A/N: When I said I'd take ages to update, this isn't really what I meant. I'm sorry. Also, last time I was vague as to where in DW canon this was - I think I'll firmly place it between The Angels Take Manhattan and The Snowmen now. And I've had this planned out for almost a year now and I've always planned to include Jack, Rose and the copy of Ten in various chapters.

"So," the Doctor said briskly, yanking on a lever and spinning on one heel to watch Sherlock. "Where to first?"

Sherlock didn't answer for a moment, too busy staring around the interior of the 1960s police-box – TARDIS, the Doctor had called it. It was a lot more cluttered than he had expected, with protrusions and shapes extending strangely for no apparent purpose. Perhaps he didn't know enough about the way it would work. The Doctor leaned against a console, almost missing and stumbling slightly. "It's bigger on the inside," he said, as though people were routinely surprised by this. Sherlock supposed they must be.

"Evidently," he replied calmly. It wasn't as though he had expected the Doctor to ask him to travel in a one-metre-squared phonebox.

The taller man looked mildly surprised and almost disappointed for the span of about half a second before he hitched his boyish smile back up and spun around again, doing something rather violent to another lever. "So!" he repeated in the same bright voice. "The whole of space is open to you. Where do you want to go?"

The World's Only Consulting Detective shrugged his shoulders bewilderedly. "I don't know," he said. "I don't know anything about space, I've never needed to. John always used to find it funny," he added, feeling his heart squeeze itself around the ever-present hole in his chest. "He used to try and teach me about the solar system, but I never thought it necessary to retain the information."

The Doctor gave him a long look. "A blast from the past, then?" he asked softly after a moment. "I could teach you, and you could remember it to keep a piece of him with you." Sherlock bit the inside of his bottom lip to prevent it from trembling. He'd followed the Doctor around the block and into the police-box with the idea in mind to forget about John. What had he been thinking? He couldn't forget. "What do you say? A first-hand informative tour of the solar system with the greatest expert the universe could provide as your personal tour guide."

There wasn't a tremendous amount of choice. Sherlock smiled. "All right," he said, trying to sound cheerful. The Doctor grinned gleefully and skipped around to spin a dial on the other side of the control panel. Sherlock tried to relax and stepped up to the platform to lean against the padded railing. "When do I get to meet this expert, then?" he teased, receiving a half-hearted swat of a disproportionately large hand in return.

"Right!" After barely a moment of disconcerting, slightly rocky motion that Sherlock could only assume was flight, they did something that might have been a landing with a rhythmic screech and a kind of ominous-sounding dong. The Doctor made a few running leaps to the door and threw it open. "Planet Mercury," he said with a dramatic flourish at the landscape outside. With a quick confirming glance at the Doctor, he stepped out of the door, the brittle dust of a different planet crunching satisfyingly under his shoes.

The Doctor started talking again and Sherlock tried to listen to what he was saying and drink in the barren, bright atmosphere at the same time, but the other man was talking in a quick, recited, perfunctory manner and it was rather difficult to keep focus. "See, what you have to understand – careful, the TARDIS shields only extend for roughly ten paces in any direction and without them you'll suffocate and burn and die."

Sherlock turned around to give his tour guide his full attention. Suffocating, burning and dying weren't on his list of things he wanted to do on this tour. "Thank you," the Doctor said graciously, making a deferential gesture and clicking his heels together. "See, what you have to understand first is that this is just one solar system, and there are hundreds of thousands more out there – the human race never manages to catalogue them all. No matter how far out into the universe you go, there's always more." He stopped to draw breath, and a giggly sort of grin snuck across his face. "Space," he began, with the sort of tone one might adopt as a headmaster in a primary school, "is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you might think it's a long way down the road to the chemist's, but that's just peanuts to space."

The smile that had been threatening Sherlock's calm and interested expression finally broke free. "I've heard that before," he interjected when the Time Lord stopped for breath. "It's from a book, isn't it? John must have read it to me. Was it the Dummies Guide to Space, or something?"

"The Hitchhiker's Guide to the Galaxy," the Doctor corrected.

"That's it," Sherlock agreed, fiercely tugging his mind away from John Watson turning to face him after one of his more oblivious comments about the solar system with that adorably incredulous smile on his face. "Is that a real book? I didn't think it was."

The Doctor smiled sadly. "No, it's not," he said offhandedly. "I tried to write it once, but the manuscript wouldn't fit into a publisher's envelope. Anyway," he added forcefully, tweaking Sherlock's lips into another smile as he attempted to reclaim the conversation. "Welcome to Mercury. Closest planet to the sun, named after the Roman messenger for the gods. He was my favourite, ol' Mercury – or at least, his Greek counterpart was. The Greeks were much more fun. The Greek messenger of the gods was also the god of thieves and tricksters and practical jokes. And music, or he was until the day after he was born when he had to hand that over to Apollo for stealing a whole bunch of sheep –"

"But you digress," Sherlock interrupted.

For a moment the man looked puzzled. "Do I?" he asked – but the question had barely left his lips before the answer was tumbling after it. "Yes – yes, I do," he almost shouted. "Mercury. Close to the sun, surface temperature…" the Doctor brought a finger to his mouth, sucked on it obscenely for a moment, then held it to the air. "Three hundred degrees at the moment, but when this side faces away from the sun it'll drop down to almost -200. Planetary mass is 5.5% of Earth's mass, diameter of 4,878 kilometres at the equator, 58.6 Earth days to one Mercurian day. No life at the moment – not even an atmosphere, but there will be in a million years or so. No," he said quickly, noticing the hopeful look that Sherlock hadn't been able to eradicate in time, "we can't go there. Even in a time machine it's too risky. See, stars that become suns, like this one," he flapped a hand at the hulking orange globe on the horizon that Sherlock had already surmised was the sun, "they have a limited lifespan like this, and then they expand into what they call 'red giants'. The Galactic Trust held this sun back with a forcefield for thousands of years to preserve and evacuate the Earth, but eventually their funding ran out and they had to let it go. Life on Mercury had quite literally just formed something that could be termed a stable ecosystem, and then…" The Timelord made a whooshing noise and a dramatic hand gesture. "The first three planets in the solar system exploded. They evacuated Mars as well, but that only ended up covered in dust. A whole world of life just… gone. Because nobody thought to check." He paused. "I was there," he said quietly. "No-one was even watching."

They stood there for a moment, staring out across the bleak landscape of the planet. Sherlock could understand why it had taken a further million years for life to form here; it was dry and crumbling and if the Doctor's mention of burning and suffocating was any indication, swelteringly hot. After such a pause the Timelord sprang into action like a clockwork mouse. "All right. Shall we continue?" he asked brightly. Sherlock gave him a wry sort of look before following him back into the TARDIS.

The spaceship was old, Sherlock realised as they lurched from planet to planet unsteadily. And it was likely the Doctor wasn't quite flying it properly; they landed with a lurch and a terrible noise and the Doctor instantly sprinted around to the other side of the console to flip a few levers. It must have been designed to be flown by more than one person, the amount of running around he constantly had to do. He wondered if he'd be allowed to stay around for long enough to learn how to help.

"Venus," the Doctor introduced with an even larger flourish, throwing the door open to let in a blast of humid heat. "We won't stay here long, even the TARDIS shields have trouble keeping the heat out."

Sherlock peered out of the door, but he didn't feel like exposing his face to the heat creeping its way into the TARDIS. "Why is it hotter here than on Mercury? Isn't it further from the sun?"

The Doctor smiled as though he had expected the question. "Mercury had no atmosphere, whereas Venus has a very dense atmosphere that actually traps the sun's heat close to the ground. Like closing the curtains to keep the warmth in at night."

He nodded slowly. "All right," he said. "Can we close the door now?"

With a bright grin, the Timelord slammed it shut and spun back to the consoles. "What do you think so far?"

Sherlock shrugged. "A lot of barren, stifling hot wastelands. I'm waiting for you to impress me, Doctor."

He watched the slow, delighted smile spread over the Doctor's boyish face. Clearly he didn't often have to try to impress people. He supposed most ordinary people would have been hooked by the spaceship that was bigger on the inside – or maybe it was just that ordinary people placed more significance on standing on the rocky surface of Mercury.

He missed John. The look on that strong face when he stepped out onto another planet would have made everything worthwhile.

"Right!" the Doctor snapped him out of it by somewhat violently flipping a lever on the console. Sherlock found something convenient to lean on and watch him. "We'll skip Earth because you've spent plenty of time there already and go straight on to Mars – the first planet colonised by the human race, you'll like this one. 2058, the first group of people built an oxygenated hub on the surface of the planet – by 2150 there was an entire civilisation on there. But we are going even further forwards than that – by the year 3000 Mars was a universally-recognised place of learning."

His square hands crept towards the control that Sherlock had guessed was the kick-off for flight; Sherlock moved forwards to stop him. "Doctor," he said brightly, "is there something I can do to help you fly?"

The lanky alien stared at him. "Pardon?"

"Well, this ship was built for more than one pilot," Sherlock reasoned. "Judging by the shape of the console and the way you're having to rush around to get us going. Probably more than two pilots, but two still must be better than one, surely?"

He smirked slightly as the Doctor still didn't move. "It's a complicated system," the Timelord said dubiously.

Sherlock smiled. "I'm a fairly quick learner."

For a moment they stared at each other, Sherlock's eyebrow raised in challenge, the Doctor frowning as though trying to decide whether he could trust Sherlock. Then there was an abrupt humming noise from somewhere above the console, and the lever that the Doctor had just nonchalantly flipped upwards slid itself back down with a resonant click.

The alien sighed. "Yes, all right, old girl," he said in a long-suffering manner, rolling his eyes but patting the lip of the console in a revealingly tender manner. "It might make it smoother, I suppose. We could try. Here, you go on this side…"

Sherlock followed the Doctor's lead, placing his hands where the lanky man directed them. The console was warm under his fingertips, and Sherlock felt almost peaceful holding onto it, like the ship itself was reaching out to him. He drew in a sharp breath at the sensation.

The Doctor smiled. "You can feel her, can't you?" he asked, his voice oddly soft.

Were it anyone else, Sherlock would have accused them of being so sentimental they had imagined it, the feeling of benign consciousness rubbing against his hands like a cat. But he was Sherlock Holmes – he hadn't imagined that the ship might be alive in some way until he had touched it and it had become obvious. "But it's a ship, a vessel," he protested, his fingers moving in stroking motions across the controls nonetheless. "How can it be alive?"

"She runs through the Time Vortex," the Doctor explained. "You try being inanimate with the whole of time and space powering your heart."

He couldn't help but smile at the metaphor, noticing the movements of his fingers but being unable to stop them. The Doctor looked down at them and grinned. "She approves of you," he said, his voice returning to its usual joviality. "I didn't think she would. I thought she might even try to throw you out, you being all…" he waved a hand like a fish fighting for oxygen, the gesture encompassing Sherlock's body. "Complicated."

Sherlock raised an eyebrow. "Complicated?" he tried to clarify.

The Doctor paused, as though he had just realised that he had let slip a piece of information he was supposed to keep clutched to his chest. "You said it yourself," he expanded reluctantly. "Your version of your timeline doesn't quite match up with the version of your timeline that I know. That makes you a complicated time event, you shouldn't exist the way you are. Last time someone like that tried to touch the TARDIS she flew almost to the end of time to try to shake him off. But you… she's even encouraging you to fly her." The Doctor stared at him for another moment, then clapped him on the back. "Stops me worrying about your timeline, anyway," he said brightly.

When the Doctor flipped the flight lever for a second time, the stabilising controls (the blue stabilisers, the Doctor had spat out like a curse as he directed Sherlock's hands to them) tingling under Sherlock's palms, the rock of movement was easy to brace against, more like the rolling deck of a cruise liner than the shudder of an earthquake simulator. Sherlock smirked at the alien around the central column. The Doctor rolled his eyes, but grinned back as they settled with the same asthmatic sound they'd made on their last landing.

Sherlock frowned. "Is it supposed to make that noise when it lands? It sounds unhealthy."

The Doctor scowled. "Of course it's supposed to make that noise." He paused for a moment as though unsure; Sherlock raised an eyebrow. "No, it's not supposed to. I just like it when it does. Are you going to criticise the way I fly my ship all day, or are we going to go outside and see where we've landed?"

Raising a mock-sceptical eyebrow, Sherlock teased, "What, don't you know where we've landed?"

The comment earned him a light slap on the arm. "Of course I know," he said disparagingly. "But I'm not trying to impress me."

This time, the door swung open onto a room that was unmistakeably a library.

Sherlock had never liked libraries back home, but he instantly appreciated this one. The alluring smell of old books pervaded the control room; Sherlock hazarded a guess that all the books in this library were positively ancient by his standards. It was comforting to know that something so mundane as a library would still exist so far away from Earth, 2015.

"Ah," the Doctor proclaimed, joining Sherlock at the door. He shifted as the tweed of the Timelord's jacket brushed against his shoulder. "A library."

He chuckled. "Yes, thank you, Doctor." At the Doctor's affirming movement, Sherlock stepped into the Martian library. "I'm surprised books still exist in this form so far into the future," he confided, running his fingers along the spine of Adelaide Brooks: The Unauthorised Biography. "Even back in my time, people are starting to worry that everything will be digitalised."

The Doctor scoffed. "No," he dismissed, flicking a far less careful hand at a row of Chinese history books. "There will never be anything else quite like curling up with a book. Maybe it's the smell, or the texture, or just the romanticism of the idea – but every time people try to make the shift to purely digital books the public put up some kind of protest. Books are still being printed like this right up until the year 4 million."

Sherlock allowed one corner of his mouth to turn up as he scanned the books down one aisle, enjoying the strange titles and antique bindings. He spotted a slightly mouldy-looking shelf of Reader's Digest classics and smirked at them. "So we're in the 31st century, then?" he asked slowly.

The Doctor theatrically licked a finger and then held it up as though testing the direction of the wind. "Yep," he confirmed brightly. "Middle of, I'd say. Just in time for –"

A door at the opposite end of the library banged open; the Doctor blanched and took a reflexive step towards the TARDIS, so Sherlock retreated to the blue telephone box as well. When the newcomers rounded the corner he started slightly at the sight of the two slim, petite women. He couldn't help but frown bewilderedly at them; what kind of outfit sent two young women to investigate intruders?

The taller blonde woman stopped in front of them, raising her chin authoritatively as she stared. Her companion, slightly shorter, tight curls spiralling in all directions around her dark-skinned cheekbones, seemed content to stand further back and peer hopefully at them around her friend.

They weren't investigating them, Sherlock realised. It was almost as if they were waiting for them.

He watched the blonde's eyes travel from the two of them to the POLICE BOX PUBLIC CALL blaring from the top of the box behind them and understood.

"Are you from the Shadow Proclamation?" she asked. "I wasn't sure if we'd got through – the signal doesn't always work here, you see."

The Doctor nodded reasonably. "The Martian Wastelands, I've heard. I can fix that for you, if you like." He brandished the screwdriver proudly.

The woman looked disappointed. "So you're like… an interplanetary electrician?"

"I'm sort of just… generally here to help," the Doctor corrected, rocking back on his heels and splaying his hands across his chest. Sherlock suppressed a smile. "But tell me – why were you calling the Shadow Proclamation?"

It was the dark-skinned girl who piped up this time, peering nervously around her companion. "You haven't heard? It's been all over the Martian news cycles." The Doctor spread his hands in a gesture to continue. The girl looked awkward. "Well… because of the murders."

Sherlock snapped his head up to peer interestedly at her, his blood suddenly humming. "Murders?" he repeated intently.

The Doctor sighed and scratched the back of his neck. "Oh, boy," he said resignedly.

The two women led them out of the library and down what felt like a million flights of stairs into a crowded hallway. Sherlock recognised it instantly as a university; young people with exaggerated spectacles and faintly ugly cardigans were hurrying through the halls in different directions, clutching multiple electronic tablets and the odd old-fashioned laptop and chatting furiously to each other. Sherlock smirked as he heard snatches of essays and deadlines and assessments that hadn't been studied for.

"I thought the Mars complex was all one-story," the Doctor said, frowning at the students as they bustled self-importantly past.

Kirsty, the taller woman with the confident stride, smiled slightly. "Most of it is," she told him, only tilting her head back in order to direct her words towards him so that most of the sound was captured by the crowd and Sherlock could barely hear her. "But the English department wanted the library up a tower. The architects had a fit, but they insisted. Said if they were going to have an Old Earth library with all the classic old books they salvaged, they wanted a proper tower room with a spiral staircase and bay windows."

Hannah, the petite dark-skinned woman, smiled shyly back at Sherlock. "Kirsty's not in the English or History department," she explained. "She doesn't understand the romanticism of an Old Earth tower library. Surely you must, Mr…"

Sherlock opened his mouth to give her his name, but the Doctor got there first. "Jones," he interjected quickly. "He's… Robert Jones. And I'm John Smith. Sorry we didn't do that earlier. And yes, we're both huge Old Earth enthusiasts." Sherlock stared at him.

Kirsty snorted. "Bob Jones and John Smith?" The Doctor shrugged sheepishly, giving Sherlock a quick shake of his head that made his hair flop into his green eyes and out again. "I don't care who you are or why you're using fake names if you can help us stop these murders. I've never seen anything like them."

"You know," the Doctor said conversationally, "somewhere in the world, there actually are people called John Smith and Bob Jones. Quite a few of them."

Hannah smiled her shy smile again; Kirsty didn't react. Sherlock thought the Doctor was rather missing the point of her declaration, and he didn't understand why they were using such obvious fake names. "Where are we going?" he asked sharply.

They had turned into a corridor that was entirely empty and ventured through a door conspicuously marked university staff only. "Maintenance," Kirsty threw over her shoulder. "Out to the forest."

Forest. Sherlock almost asked her what she meant, but the Doctor looked understanding, so he thought about it instead. Oxygen supply, he reasoned after a moment.

They walked for another few minutes; Sherlock guessed that the forest would be right on the edge of the complex, assuming by the way she had said it that they only had one forest to power the school. He casually slowed his pace a little so that he and the Doctor were walking a few steps behind the two women.

"Why are we using fake names?" he asked out of the corner of his mouth.

The taller man stopped and yanked on Sherlock's sleeve, forcing him to a stop as well, his shoes skidding slightly on the smooth floor. "When I first met you I recognised your name, remember? You're a part of history, and these people – well, Hannah – study history. If you weren't supposed to be in 21st century Germany, that's nothing compared to how much you're not supposed to be in 31st century Mars."

Sherlock blinked. "Of course," he agreed. In fact, he should have been able to come up with that on his own. The fact that so much of his surrounding was unfamiliar, that there was so much of this world that he did not understand, was extremely distracting.

"You two okay?" Kirsty called back, raising an eyebrow and making it quite plain that she didn't really care whether they were okay or not.

The two women had stopped outside another door and were watching them expectantly. Sherlock raised an eyebrow. "Fine," he replied archly. "Are we going in?"

Kirsty pursed her lips as though she had just bitten into something sour. "When you're ready," she said, twisting the doorhandle rather more sharply than necessary.

Sherlock wasn't sure what he had expected from an oxygen forest, but the white domed ceiling definitely wasn't it. The walls of the place curved steeply upwards into a huge dome that must have looked fairly spectacular from above, arching out of the red soil. More unnerving, however, was that the light in the dome seemed to be emitted by whatever the walls were made of, casting a slightly unhealthy, artificial pallor on the great trees. Other than that, it looked like a conventional 'Old Earth' forest, like it had been transplanted straight from the middle of rural England.

Kirsty led them down a path to one side of the forest, dead leaves crunching under their feet, and up to a rustically lichen-covered sort of hut up against one wall. "We kept the… bodies… in here," she said, frowning at them and looking almost apologetic. "We thought the Shadow Proclamation might need to look at them, you know, when they finally turn up."

Sherlock frowned. "They're not decomposing in there? Doesn't the smell…?"

The woman's mouth thinned. "That's not really a problem," she said softly, jiggling a key in the lock and throwing the shed door open.

"Oh!" the Doctor exclaimed jovially. "Talk about skeletons in your closet, Kirsty."

Sherlock tried to give the alien a wryly amused look rather than laugh out loud. This fake forest was making him slightly nervous. "How long ago were these people murdered?" he asked instead, stepping closer to examine the ancient-looking skeletons hanging on twin hooks from the cupboard wall.

Kirsty and Hannah were hanging by the door, staring apprehensively at the skeletons. "The first one was about three weeks ago."

He allowed his fingers to probe gently at the completely dry, fake-looking bone. "What could do this to a person in three weeks?"

"Actually," Kirsty interjected, "we found them like this. Looking like they'd been dead for years and preserved like this. Whatever it was it can't have taken more than a few hours to do it."

The Doctor unfolded his arms and stood up from where he had been leaning against a dusty workbench. "Oh, I'd say it wouldn't have taken more than a few seconds," he said ominously.

Sherlock, Kirsty and Hannah stared at him. "But what –"

"We thought maybe it was some kind of machine?" Kirsty cut over Sherlock's admittedly pointless question.

The Timelord looked at her sharply. "Who would invent a machine to do this?"

Kirsty stuttered a little, glancing at Hannah. "We… we thought because of the plague on the Degavar System, you know, they might have had to invent something like this in the morgues…"

"Right," the Doctor said, nodding slowly. "What year is it?"

The librarian stared. "3074," she replied, sounding nonplussed.

"Oh, that plague," the Doctor said vaguely, his eyes sliding from the skeletons to Sherlock, who stared back. "No-one's actually died of that plague, I don't suppose the news will have come back to you yet out here." Kirsty and Hannah exchanged bewildered looks; the Doctor breathed in sharply and stepped away from the skeletons. "No, I don't think this was done by a machine."

Something moved in his peripheral vision; reflexively, Sherlock stepped away from it. He looked guiltily around to see if anyone had noticed the movement when he realised that he had jumped at the shadow of the nearest tree. The Doctor gave him a sideways glance, but didn't comment. Sherlock tried to force himself to relax; there was something about this artificial forest and its artificial light that set his teeth on edge.

He looked around at the dome's eerily lit curve; the way that the light seemed to be emitting from the fabric of the dome itself rather than from visible lights must take some getting used to from the people who worked here.

Something about the light stuck in his mind; a tiny nagging sensation that made his face scrunch up and directed his eyes back to the shadows of the trees stretched across the ground. Then it clicked.

There shouldn't be any shadows.

If the dome cast light from every direction, there should be an equal amount of light hitting the trees from every angle and no place for a shadow to fall.

Sherlock crouched to examine the shadow. It wasn't even as though it were a faint suggestion of a patch of less light; it was a definite shadow that fit the shape of the tree above so well that he hadn't noticed that it shouldn't be there. Wondering whether it was cold in the way that ordinary shadows cast by the sun back on Earth had been, he stretched out a hand towards it –

"Sher - Bob."

He looked up at the Doctor's sharp cough and note of warning, automatically withdrawing his hand. The Timelord was watching him with a heavy frown over his eyes. "Keep away from the shadows," he said in the same terse voice so unlike the boyish roll he'd heard so much of previously.

"But there shouldn't be any shadows," Sherlock protested. "The light in here is coming from all directions, there's nowhere for a shadow to fall."

The Doctor glanced uneasily at the offending shadow. "Exactly," he said unhelpfully.

Sherlock stood up and moved away from the tree, returning to the Doctor's side. "Okay," he said slowly. "Explain."

"Did you know that almost every species in the universe is afraid of the dark?" the Timelord said cryptically.

Sherlock refrained from rolling his eyes. "Yes," he said shortly. "It's an inherent, irrational paranoia of the unknown."

The Doctor shook his head with the tiniest of regretful smiles. "No," he corrected. "It's a perfectly rational fear of the things that live in the dark."

He couldn't quite stop the thrill from racing up his spine, irrational as the things the Doctor was saying was. He rolled his eyes to counteract the effect. "Nothing lives in the dark, we'd know."

"Really?" the Doctor countered, lifting a faint eyebrow. "You never heard of anyone who just wandered into the shadows and never came back? Never wondered about the things that go bump in the night?"

Sherlock did roll his eyes this time. And yet – there had been James Phillamore all those years ago, went to get an umbrella and never came back; people just disappeared all the time, when he thought about it. "All right," he said slowly. "What is it?"

The Doctor pulled out his screwdriver and pointed it briefly at the shadow. "It's Vashta Nerada," he said heavily.

Sherlock frowned. "Bless you," he suggested slightly incredulously.

To his surprise, the Doctor turned around and snapped at him. "It's not funny," he said sharply. "The last time I got stuck in a place infested by an aggravated swarm of Vashta Nerada, someone very important to me died."

The following silence hung heavily between them; the two women shifted awkwardly, looking terrified. "Okay," Sherlock accepted. He would have to adjust to the fact that he would have to take the Doctor's word on so many things in this environment as read, as dangerous as it felt to leave so much in someone else's hands. "What do we do?"

"Evacuate," the Doctor said immediately. "Seal off the forest and never open it again. All of the deaths have been inside this room, right? There's no reason to assume any of them have spread beyond the forest?"

Kirsty shook her head. "No, everyone that's died has been in the forest. But we can't just seal it off. We'd have no oxygen. We can't get any kind of replacement immediately."

"But surely the process is automated somehow?" Sherlock protested. "It will take years for the forest to die, even if you don't give it food or water – both of which I would assume to be possible without entering the forest itself. You wouldn't need the replacement immediately."

But the woman was still shaking her head. "It'd be much less efficient, there's no way to guarantee we'd be able to harvest enough oxygen externally to supply the entire university on a day-by-day basis. And even then, to grow an oxyforest like this takes years. We'd have to get a fully-grown forest shipped in from Earth, which is a beaurocratic nightmare with the conservationists down there –"

"It's that, or people will die in here!" Sherlock retorted, his voice rising to a shout. "You don't really have a lot of options right now!"

"Kirsty," the Doctor interjected suddenly. "Can you take a step forwards, please?"

Sherlock looked at the woman's feet; with a thrill, he noticed that the shadow from the nearest tree had advanced on her, as though stretching with the sunset, and was now lapping gently at her ankles. Kirsty, too, looked down and made a frightened squeaking noise. She stepped slowly and deliberately forwards; to Sherlock's horrified amazement, a shadow split off from the tree-shape in the form of her shapely calves.

She had a shadow.

"I'm sorry," the Doctor said quietly, stretching out a hand in supplication but very carefully not touching her, biting at his lower lip with his green eyes sad. "I'm so sorry."

She turned terrified brown eyes up to him, wide and pleading. "What's going to happen to me?" she asked. "Doctor?"

The Doctor's mouth was open, but he was shaking his head in a desperate inability to answer when it became apparent; Kirsty threw her head back and emitted a terrible scream. Sherlock watched, his own mouth falling open, as something dark seemed to consume her from the feet upwards. Her knees buckled; for the barest of instants, her body gleamed wetly red with exposed blood and muscle before it crumpled to the ground, stripped completely bare like the skeletons ironically-placed in the cupboard, her clothes reduced to shreds of striped fabric. The grotesque jaws of her surreally-white skull made a terrible clack as the final impact snapped her mouth shut.

The three of them stood and stared, too shocked to move. Then Sherlock recovered himself a little and looked up at the Doctor. "What now?" he asked quietly.

The alien was frowning at the pile of bones with a frightening kind of fire in his green eyes; at Sherlock's words he looked up as though surprised he was still there. "Run," he replied urgently.

And then he was off; Sherlock barely had time to register that the lanky creature had grabbed his wrist before he was yanking on it, pulling Sherlock after him back towards the door. Sherlock glanced back to make sure Hannah was following them before putting on an extra burst of speed to catch up with the Doctor. "Keep out of the shadows!" the Timelord called back.

They sprinted back to the door they had entered the forest through, Sherlock's heart throbbing in his throat. The moment Hannah was through the heavy door the Doctor slammed it shut and leaned against it, panting. Sherlock doubled over and tried to breathe through the image that wouldn't leave his head of the shadows stretching greedily towards their feet as they ran past. Hannah was gasping in shock, spluttering against the wall.

"Kirsty," she choked out after a moment.

The Doctor reached out for her arm. "I'm sorry," he told her. Sherlock couldn't bring himself to be too upset for the blond – she hadn't struck him as the most pleasant of people – but he did feel sorry for Hannah, who had obviously worked in fairly close proximity to her for quite some time, not to mention the abrupt and frankly terrifying manner of her death.

He still wasn't sure he should believe his eyes. What had happened to Kirsty should not have been possible, let alone the cause of it if he were to believe the Doctor. And yet – he was on Mars in 3074 and he had arrived there in a gigantic spaceship designed to be flown by a group of aliens but disguised as a 1960s police phonebox.

What it came down to, really, was whether he trusted the Doctor, who had taken him there and was his only source of information in this entirely new dimension of Sherlock's universe. And for a collection of reasons he didn't entirely understand, he did trust him.

"Hannah, can you take us to… the person who co-ordinates the oxyforest? This needs to be sealed off right now. There is no other way of stopping them, it can't be that hard to automate the forest so that no-one has to go in there."

The woman's shuddering breaths calmed down slightly. "Yes. Of course, I'll… take you to the Faculty Manager."

"I don't understand," Hannah continued as they set off back down the Maintenance hallways. "We've had people in there for at least a year now, gardeners and such. If there have always been Vash… whatever they are in there, why did people only start dying last week?"

The Doctor shook his head. "They might have only just hatched. Or it could just be… in ordinary forests, the Vashta Nerada survive on roadkill and the natural deaths that just happen in a forest. Maybe they didn't need to hunt until three weeks ago, but this forest is too artificial to have its own ecosystem besides the trees."

Hannah nodded. "That's why we need the gardeners, to feed the trees. That might be tricky. I don't know how these things work, of course, but I should think there's enough food in the soil, you know, for them to just be watered for at least a few weeks until we can work out some kind of system. The water is an automated system, I remember when they put it in."

The faculty manager's office was down a seemingly never-ending corridor practically straight opposite the maintenance hallways. The main corridor was almost deserted compared to the swell they had had to fight through the first time. Sherlock checked his watch to see whether they had caught the hallway right between lectures, but unsurprisingly the device was still resolutely telling him that it was almost midnight on the 5th of May 2015. He'd always wondered what happened to the wristwatches of people who time-travelled as a child. Apparently they simply didn't notice.

"Who the hell are you?" the manager blustered when they burst in, stowing something that looked a little like Sherlock remembered Playboy magazines hastily in a drawer. He smirked.

The Doctor placed both hands on the manager's desk and leaned over it threateningly. "I'm the Doctor," he introduced, apparently deciding that the time for atrocious fake names was past. "And you've got a problem."

The faculty manager looked around the office helplessly, frowned heavily at Sherlock, who frowned back, and then latched on Hannah. "Professor Sainsbury, isn't it? What is the meaning of this?"

Hannah coughed nervously. "Sir, they were here about the murders. There's been another one – Kirsty Harding's dead." Her voice broke a tiny bit on the last word, but she lifted her chin firmly. "These men know what did it, they say we're all in danger."

The manager frowned at them some more. "Well?" he asked the Doctor imperiously.

"Have you heard of the Vashta Nerada, Mr Fitzherbert?" the lanky alien asked, leaning back in order to read the manager's name from the block in front of his desk. "They're flesh-eating creatures that hatch from trees, and you've managed to import an entire swarm of them from Earth in your oxygen forest. You need to stop people from going in there or more people are going to die."

Fitzherbert's pudgy face purpled slightly. "Flesh-eating creatures that hatch from trees?" he repeated incredulously. "On Earth? You don't believe this nonsense, Professor, do you?"

Hannah shrugged. "I wouldn't sir, if I hadn't seen them… they stripped Kirsty's body to the bone. We need to seal off the forest and get in touch with the Martian ministry back on Earth."

Sherlock winced; telling Fitzherbert how to do his job didn't seem like something the man would react well to. Indeed, his face turned a darker shade of purple, but the suggestion of stripping to the bone made enough of an impact that he did not comment on it. "I need access to the system that controls the water supply to the forest," the Doctor pressed. "We must be able to feed the relevant nutrients to the forest through there and seal it off before any of them escape through the campus."

Fitzherbert's eyes narrowed. "Marchbanks!" he barked. "You're not going down there alone. Someone needs to keep an eye on you."

The door to the left of the desk swung open and a young red-headed man stepped neatly into the room. "Mister Fitzherbert," he replied politely. "How can I be of assistance?"

The corner of the Doctor's mouth turned up as he looked at the evidently efficient secretary, like he reminded him of someone. Fitzherbert looked unimpressed. "I want you to take this man down to the basement to look at the water systems that feed the oxyforests. Take a security team down with you, make sure he doesn't touch anything. I still don't trust him."

Marchbanks dipped his head politely. "Hannah and I will do it," Sherlock interrupted quickly. The Doctor looked around in surprise. "Hannah and Mr Marchbanks can fill me in on anything I might be unfamiliar with in terms of the technology and I'm sure we could work something out. At least get a sense of what needs to be done. And then you can stay up here with Fitzherbert – I'm not about to do anything to your systems while my colleague is still here with you, so the security team is unnecessary."

Hannah smiled weakly at him; Fitzherbert harrumphed pompously. "Very well," he snapped, although his show of reluctance was very poor indeed by now. "Marchbanks, watch them like a hawk. Any sign of tampering with the equipment and you call security." Sherlock rolled his eyes.

The secretary smiled lightly. "Of course, sir," he said briskly. "If you'd like to follow me, Professors."

Sherlock gave the Doctor a shrug on his way out the door; the alien smiled in response, clearly having worked out Sherlock's motives. The Doctor knew more about the situation and therefore would have more luck with Fitzherbert, not to mention the manager's obvious approval of the idea of having a hostage to make sure Sherlock didn't touch anything. Sherlock, however, valued his ability as a negotiator. Marchbanks seemed like he would see the reason behind fiddling with the water systems and not run screaming to Fitzherbert until the Doctor had managed to gain his consent.

"So," Hannah piped up as they walked. "Where are you two from, anyway?"

Sherlock let the corner of his mouth twitch upwards. "A long way away," he said truthfully. He wasn't sure how else he was supposed to respond to that; he didn't even know where the Doctor was from. He'd repeatedly stated that he wasn't human, but what did that make him? And how was he supposed to tell her he was from the twenty-first century and, apparently, a name that she would recognise?

She laughed softly. "Right. Bob Jones from a long way away. You two really don't want anyone finding out who you are, do you?"

Sherlock shrugged. He supposed when this was over there would need to be a discussion about that – trouble seemed to follow the Doctor with surprising alacrity. Or perhaps it was the other way around, remembering the way he had entered the bank in Frankfurt because the Judoon were already there. Sherlock was not complaining either way; he hadn't had this much fun since he had left Baker Street.

Marchbanks led them down a few corridors, then stopped and turned to Hannah. "What's going on that you need access to the forest water supply, Professor?" he asked. Hannah looked at Sherlock uncertainly.

He cleared his throat. "Mister Marchbanks, your oxyforest has hatched a murderous swarm of creatures called Vashta Nerada. They look like shadows – look so much like shadows that you don't even notice they shouldn't be there. I presume you've heard about the skeletons in the oxyforest? That was them."

The secretary smiled tightly at him. "Right. And you want to check the water systems to see if there's a way we can seal off the forest completely to stop it happening again, but Fitzherbert doesn't want to get the Martian Ministry involved, start the process to get the forest replaced?" Sherlock frowned at the young man, surprised at his speed. Marchbanks grinned. "That man is a beaurocratic nightmare. I'll see what I can do for you."

Sherlock smiled at him, tucking his hands into his coat pockets contentedly. "Well, that was easier than I expected," he admitted wryly. The other man just quirked a smile back.

The basement was dark; Marchbanks flicked a bank of lights on and the darkness skittered away into the corners. Sherlock cast a glum look around. "Of course," the secretary muttered, more to himself than the others. "Murderous shadows and we have to stick around in the dingy shadowy basement."

Sherlock rolled his eyes. "We don't think the shadows have spread outside the oxyforest yet." His eyes fell on a series of pipes and controls, what looked like a row of pressure dials and some kind of intercom system. "Is this the water system to the forest?" Marchbanks nodded. "And that intercom, can we reach Fitzherbert's office from there?" Another nod. Sherlock nodded sharply. "Right. Are either of you two familiar with how this works?"

Hannah shrugged helplessly, but Marchbanks tipped his head thoughtfully. "I know the basics, but I've never touched it before. It's a simple system. Let's take a look, shall we?"

They crossed the tiny room to the pipes and Marchbanks bent to examine the pressure gauges. Sherlock followed, running his eyes over the lengths of the pipes to where they disappeared into the ceiling. As he watched, the overhead lights flickered and went out.

Sherlock froze. His entire body crackled with fright before he schooled himself back into calm – it was just the lights playing up. He wasn't a superstitious person. Hannah, however, squealed in alarm. "What's that?" she squeaked, her hand finding Sherlock's arm in the darkness and clutching it painfully tight.

He put his hand over hers and tried to gently pry her fingers away from his arm, but she tightened her grip and he gave up. "It's just the lights, Hannah, don't worry," he reassured her. "Do they play up often, Mr Marchbanks?"

There was a clang from the other side of the room, a muffled curse, and then a flashlight flickered to life. "Often enough that there are always flashlights in the basements, I suppose," Marchbanks said from behind it. "I don't really know. I don't come down here often."

"Right," Sherlock dismissed, turning back to the pipes, trying to shake off the residual tingles of unease as Hannah very slowly released his arm. "Can you get Fitzherbert's office on the intercom first, we'll see if they can get someone to fix the lights."

The secretary tossed him the flashlight – Sherlock caught it out of sheer luck, not expecting the throw – and crossed to the intercom speakers, quickly locating a keypad and typing in an extension number.

"Doctor?" Sherlock called into the speaker before thinking it through. "Doctor Smith and Mister Fitzgerald, are you there?" he covered quickly, carefully not looking at the others.

The speaker crackled. "Of course we're here." It was the Doctor's voice, not Fitzgerald's. "Everything all right down there?"

Sherlock looked at Hannah and Marchbanks. "We're fine, except that the lights have died. They came on at first and then they sort of flickered and went out – but we found a torch, so we can still see to some extent. It'll just take a bit longer to –"

The torch flickered, winking out for a moment before shining again as though nothing had happened. Sherlock paused to look at it. It was odd - first the lights, and now the torch. The creeping feeling of unease he had been feeling since the lights went out intensified.

"Are you all right?" the intercom crackled.

Sherlock shook himself. "Yes, of course, sorry. The torch just went out for a moment, it's fine now."

There was a ringing silence at the other end of the line. The torch flickered again for a few moments before dying completely. Sherlock's skin crawled. "The lights," the Doctor said quietly after a minute or two. "Did they come on when you first went into the room, and then die like the torch?"

Marchbanks gave up tapping the face of the torch and frowned. "Yes, they did," he replied, sounding surprised. "I thought it must happen all the time, because there was a torch down here by the door."

The Doctor made a nervous, nasal noise. "It's the Vashta Nerada. Get out of there, all of you." Hannah squeaked, her hand finding Sherlock's arm again in the dark. Sherlock patted it gently, trying to comfort her even though his skin was crawling under the heavy fabric of his overcoat. "Mister Fitzherbert," the Doctor continued, "you need to make that evacuation order now. If they're in that basement, it's only a matter of time before they spread throughout the campus. You've already lost four of your staff, please stop this before anyone else has to die."

Sherlock heard the faculty manager sigh. "All right. All right, I'm doing it. Just get Marchbanks and Professor Sainsbury out of there."

Unexpectedly, both torch and lights all of a sudden relit, flickering once before shining as though nothing had happened. Sherlock blinked up at the overhead lights. "Oh," he said absently. "The lights are back on. Maybe it was just a -"

"Check your shadows," the Doctor insisted abruptly. "Everyone make sure they only have the one they're supposed to."

Sherlock shot Hannah a small smile before looking down. The lights overhead were casting several pale shadows extending out to one side of him like petals on a flower. None of them were as dark as the one Kirsty had had in the forest, and none of them were the reason for his sharp intake of breath and sudden inability to think rationally.

There was also a longer, darker one to his other side, the side next to the wall, the side from which the lights were hitting him. "Doctor," he breathed. He quickly checked the other two, but they only cast the expected shadows from the lights.

"What?" the Doctor asked, his voice suddenly breathless with concern. At the renewed sound of his voice, Sherlock snapped back into sensibility. "Do you have two shadows?"

Sherlock swallowed, looking up at Hannah and Marchbanks. They had both taken note of his situation; Marchbanks was stepping surreptitiously away from him and Hannah was staring, wide-eyed and fearful. He took a deep breath. "Both of you, get out of here," he said calmly. "We know they're in this room, get back upstairs and get out of the building with everyone else."

Hannah swallowed. She looked absolutely terrified, and yet Sherlock could tell she was about to refuse, to insist on staying with him. "You can't help," he said before she could say anything. "I can get out of this, I promise, but I can't promise I could get you out if you stayed and they got you, as well. You need to leave." He wasn't sure who he was trying to reassure that he could get himself out, but all three of them seemed to relax slightly once he'd said it. Hannah gave him a tremulous smile.

"I'm holding you to that promise," she said softly. Sherlock smiled back, aiming for reassuring but feeling slightly derisive - it wasn't as if she could do anything if he didn't. She stepped forwards as if she was going to embrace him, so he stepped back hurriedly. "Right," she said softly. "Yes. Probably shouldn't."

He gave an apologetic shrug as if he'd wanted her hug, secretly grateful to have an excuse. People were always trying to hug him - usually family members of victims - and finding an excuse to shrug them off was generally something John had done for him. Or not, if he was annoyed with Sherlock and wanted him to suffer a little. A tiny smile wormed its way onto Sherlock's face. "Go," he repeated, gesturing the other two towards the door.

Smiling nervously back at him, they went. Alone in the basement, Sherlock stared at his extra shadow. "Doctor?" he said after a moment.

"Still there, then?" the voice answered. Sherlock considered making some kind of John-like sarcastic comment about where else was he going to go, but he knew that that wasn't why the man had said it and the knowledge kept him quiet.

"Yes," he replied instead. "What can I do?"

The Doctor sighed, sounding endlessly frustrated. "Hold very still. Let me think."

Sherlock complied, his own mind racing while his body froze uncomfortably still. Kirsty had died within moments of the swarm attaching itself to her. By those standards, he should already be dead, so he couldn't count on having time. Relying on the Doctor having some alien way to save him went against every fibre of instinct he had. There must be some logical way to get rid of the shadow.

"Still there?" the Doctor asked. His voice trembled a little.

Sherlock didn't roll his eyes. "I'm still here," he said quietly. "You said you'd encountered the Vashta Nerada before. How did you get rid of them then?"

"I didn't," the alien's voice was just as soft as his own, a distinct note of resigned apology in it that made Sherlock's scalp crawl uncomfortably. "We evacuated. Only one of the investigative party survived." There was a tiny pause, and then the Doctor added even more quietly, "I'm sorry, Sherlock."

He wanted to pace, but the Doctor had said hold still. His fingers twitched restlessly, as though itching to fasten around a cigarette or a syringe. He clenched his fists. "I will not die here," he said clearly through gritted teeth. "I won't."

There was a pause. Then a sound like someone clapping abruptly. "Okay then," the Doctor said brightly, the defeatist tone gone from his voice. Sherlock could picture him jumping up and flexing his fingers in front of him until they cracked. "Let's get you out of there. Invulnerable swarm of Vashta Nerada spreading through a university campus and minutes before I cause the premature death of Sherlock Holmes - but I've still got my screwdriver and I've still got my TARDIS. And I've got Sherlock Holmes. We can do this."

Sherlock did roll his eyes this time. "When you're ready, Doctor," he said idly. "They must have a weakness. Nothing is invulnerable."

"Yes," the Doctor agreed. His voice was growing louder and fainter by turns as he grew closer and further from the speaker. Sherlock envied his ability to pace. "Except that they're shadows, it's not like we could just run them through - Fitzherbert, have you put out the evacuation order yet? The fewer people I have to save from this the easier it'll be."

The shadow wasn't moving. Sherlock wondered if that was good or bad. "They're shadows," he repeated. "If I want to make a shadow go away, I cast light on it. Obviously that doesn't work, or they never would have existed in the oxyforest. Still, though - what about fire?"

The Doctor tutted. "How would we light the shadow on fire without burning you?"

Sherlock sighed. "All right. What if - "

The speaker crackled and the Doctor cut out, overlaid with an announcer's bing-bong and Fitzherbert's voice. "Ladies and Gentlemen in the Brooks Campus," it said clearly. "A technical fault has occurred with the oxygen forest supplying the building and evacuation is required. Students and staff in the west wing of the building, you are in immediate danger from the fault and your evacuation is extremely urgent. Please proceed to the nearest exit and seal all doors behind you. The rest of the building needs to be cleared entirely and all exits sealed by five pm Earth time when the university closes. Thank you for your cooperation. The main building of the university is not affected by this fault. Please return there tomorrow to check whether your classes have been cancelled or rescheduled."

The message ended with a similar bing-bong and the Doctor's voice returned immediately. "Sherlock?"

"Still here," he repeated grimly, trying to sound flippant and failing. His heartbeat was loud in his ears. Why was he still there? Was it holding still that was working, confusing the swarm somehow? Or was it something in his body that they didn't like - or something in his attitude? Kirsty had been terrified, had looked to the Doctor for help. Sherlock had smiled a grim smile at the fact that she had been so skeptical of their usefulness in solving the murders until it was her in danger. "I still think the answer is something to do with light," he mused aloud. "Maybe if we had something bright enough…"

The Doctor made a thoughtful noise. "There's a chance… if we got you back to the TARDIS I have things - enough light could make them let you go, and then it's just a case of trapping them and putting them somewhere they can't hurt anyone. But we still have an entire swarm in this building, we don't know how far they've spread. I'd hoped we'd managed to trap them in the forest, but apparently not."

Sherlock hummed agreement, looking around the basement room for inspiration. Nowhere else in the room looked unusually dark, and the lights had not flickered again. "I think the only ones in this room are the ones on me," he told the alien. "There are pipes and things in here for the water system - direct links from here to the forest. Given that there weren't any outside the forest upstairs, or they would have attacked us, can we assume that the only ones outside the forest are currently latched onto me?"

The Doctor was silent for a moment. "Probably. Ideally, we need to do a thorough sweep of the building for them before taking any kind of action, but…"

"I don't have that long," Sherlock finished for him. "They're evacuating the building. We should be able to get a few people - Marchbanks, the secretary, seemed intelligent - to check everything over if we get rid of the swarm in the forest, before they reopen the building. And if we can't get rid of them, they won't reopen the building, and then it doesn't matter where they all are."

He wondered whether he should attempt to leave the basement; he'd certainly feel a little more secure closer to the Doctor and the TARDIS, upon which apparently he could get rid of his parasitic shadow. And yet there would be so many people between here and the manager's office in the rush to evacuate the building, and he could not risk bumping shadows with any of them, given how Kirsty had gained her own shadow by stepping into the one cast by the tree. If the Vashta Nerada got outside the building, the entire effort would be futile.

"Right," the Doctor said. "Right. Last time the Vashta Nerada were throughout the entire planet, and we managed to evacuate thousands of people because I negotiated twenty-four hours from the swarm to teleport them out."

Sherlock frowned. "Negotiated? So they can talk, they can reason?"

An ironic noise issued from the speaker. "In a manner of speaking. The Vashta Nerada killed a number of people who were in spacesuits, and they managed to hold the spacesuits up and use them to move, and use the ghosting communicators to speak. These ones won't be able to do that."

A sudden vision of a skeleton in a spacesuit walking jerkily around like a cartoon mummy drifted into Sherlock's head. He shook it uneasily. "But how did you negotiate the time?" he pressed.

The Doctor coughed. "We were in a library. They hatched from the pages of the books in the library. I told them to look me up in their books before they made me any angrier than they already had, and they retreated."

Sherlock snorted. "A fearsome read, are you?" he asked dryly. It seemed rather ludicrous that the boyish, floppy-haired alien could inspire fear in such monsters as the Vashta Nerada.

"Hey," the man warned, though Sherlock could hear the smile in his voice. "I'll have you know that on some planets, the word doctor means "great warrior". In Dalek history they call me the Oncoming Storm."

Sherlock didn't know what a Dalek was, but he shrugged nonetheless. "Very impressive. But will that work this time? This swarm didn't hatch from books, they won't know who you are."

The Doctor laughed a little. "I could try listing a few of my previous achievements," he said reasonably, "but I'd have to actually do something impressive this time to prove to them that they should fear me. Last time I'd already done a fair bit to try and save the others, so they believed it when they read the books."

"Hmm," Sherlock grunted. "We could still set the forest on fire. Destroy their home and then -"

"And then make them really really angry," the Doctor finished. "I doubt we'd get a chance to intimidate the swarm around you before they killed you just for retribution." Sherlock looked down at his extra shadow and frowned at it. It hadn't killed him so far. It had had plenty of time. He couldn't help but feel as though it was waiting for something. "But," the Doctor said thoughtfully.

Sherlock smiled faintly. "But?" he prompted.

A smile was evident in the Doctor's voice when he spoke again. Sherlock felt another of those strange rushes of affection for the alien. "But, I might be able to trip the lights in the forest, turn them up blindingly high. It certainly wouldn't kill them, but it might hurt them enough to make your swarm let you go and make them willing to listen to me."

"Brilliant, let's do that," Sherlock quipped in a falsely cheerful voice.

There were murmured voices through the speaker – the Doctor arguing with Fitzherbert, no doubt. Sherlock sighed at the Doctor's muffled shout of, "Right! I'm coming down!" obviously aimed at the speaker, and then wondered whether it was worth sitting down. He'd been standing rigidly still ever since he'd discovered his extra shadow, and the phantom 'restless leg syndrome' was beginning to set in. He allowed his brain to wander off on that tangent – what was it about knowing that your body is not allowed to twitch, move, or blink that made it want so desperately to do so?

The office door opened and closed through the speaker twice; Sherlock frowned at the second time when Marchbanks' voice announced his arrival. "Is there anything we can do, sir?"

"I thought you two were getting out," Sherlock interjected pointedly.

Hannah laughed. "We're safe in this office, right?" she asked.

Sherlock rolled his eyes. "Probably," he answered. "But we can't know for sure. All three of you should be leaving the building."

"As long as there's something we can do to help here, I'd rather not get squashed by panicky students trying to evacuate,"Marchbanks said brightly. "Where's Doctor Smith?"

There was a clatter outside the basement door and the Doctor stumbled into the room as though he had run down the stairs, his screwdriver in his hand. Sherlock smiled tightly at him. "He's down here," he told the secretary. "We have an idea, but we're not sure it'll work."

"Anything we can do?" Hannah asked.

The Doctor clapped Sherlock on the back, carefully avoiding stepping on the shadow, and leaned towards the speaker over his shoulder. "Not yet, thanks, Hannah," he said cheerfully, "but if there is we'll let the two of you know." He flipped the screwdriver airily through the air and caught it one-handed. Sherlock gave him a stern but amused of-all-the-times-to-show-off look. The floppy-haired alien grinned at him. "Right," the Doctor said, his eyes scanning the control panel. "Ideally, I'd like to be closer to the forest. So we can actually tell if we're having an effect on them." He looked down at Sherlock's shadow. "Feeling all right?" he asked curiously, frowning at it.

Sherlock assumed that the question was directed at him, rather than the shadow. "Fine. Really, just like I don't have a swarm of flesh-eating shadows attached to my feet."

The man nodded. "Interesting. I've only ever seen them keep prey for this long once before. Then, I thought I'd foiled them by darkening the visor of her spacesuit so that they thought they already had her – though I thought later that it might just have been to make us forget about them and then take her when we wouldn't notice."

"What, you think they're keeping me hostage?" Sherlock clarified. "I suppose that could be a good thing." People usually didn't kill their hostages, so long as their demands were complied with. Of course, the Vashta Nerada weren't people, and he and the Doctor did not intend to comply with any demands they might give, but Sherlock was hopeful nonetheless.

The Doctor hummed absently. "Marchbanks, are there controls for the lights in the forest a bit closer? I'd like to be able to gauge their reactions to what we're doing." He paused, then added, quieter, "And I'd like your extra shadow to be able to judge it as well," to Sherlock.

Marchbanks cleared his throat happily. "Yes, there's an observation room up there. No-one's really used it since we put the water systems in, but the lights and the PA system still work."

"Perfect," the Doctor replied brightly. "Find the keys and meet us up there, would you?"

Sherlock frowned at the Doctor as he clicked off the speaker and turned to leave. If they were correct, and the Vashta Nerada were only hanging around his ankles as a scare tactic, moving couldn't do Sherlock any more harm than staying terribly still. "You could open the observation room with your screwdriver," Sherlock told the Doctor suspiciously. "Why do we need them to meet us there?"

"We don't," the Doctor shrugged. "But they want to be helpful. It can't hurt to have a couple of extra people there."

A wryly amused smile wove its way onto Sherlock's lips. He strongly suspected that the Doctor wanted them there as an audience for any clever thing he happened to do while he was up there. He'd noticed back in the bank on Earth the Doctor's flair for drama – not that he didn't sympathize. He remembered saying it to John when they had first met: that's the frailty of genius, John, it needs an audience. John had replied with a dry yes that had clearly implied Sherlock's own frailty, but he had never refused to accompany Sherlock on a case. He had to admit it was far more satisfying to present a dramatic conclusion to a case and have someone stand by and say, that was brilliant.

The shadow moved with him like any normal shadow would, limbs swinging as he climbed the stairs back to the main floor of the building. It didn't change position along with the lights, but trailed along behind him as though struggling to keep up. The Doctor was speaking to him, but he was preoccupied with staring at the shadow as it flowed up the stairs and down the hallways. It just looked so normal.

Sherlock had thought that they might have some trouble finding the observation room considering that it had been out of use for years, but when they got to the Maintenance corridor Kirsty had led them down – only hours before, Sherlock's brain reminded itself – the staircase around the corner was firmly signposted along with the warning, supervisory staff only. The Doctor grinned at him, running his fingers over the sign, before dashing up the stairs.

Hannah and Marchbanks were there already, standing by the door; the redheaded secretary had one hand on the doorknob, an impressive ring of keys dangling from the keyhole underneath. As they approached, he smiled at them and twisted the doorknob inwards. Sherlock frowned at the dusty room inside – even the huge window looking into the forest was so dusty he could barely make out the shape of trees beyond.

The lights, too, were weak, but they held without flickering. The Doctor waved his screwdriver around the room as though it could detect the Vashta Nerada, then stepped into the room, apparently satisfied. Sherlock followed, aware of Marchbanks and Hannah stepping in after him. Hannah coughed lightly as the Doctor swept a sleeve over the desk of controls in front of the window, stirring dust into the air.

"Is there a cloth around somewhere?" the lanky alien asked briskly. "So we can clear the dust off the window? This will be a lot easier if we can see them. And they can see us," he added as an afterthought. Sherlock bent and fished out a folded and itself ridiculously dusty towel from under a desk much like the ones they had had in his primary school. The rubber cap on the end of one of the legs was even missing, he noticed as he bent. The desk swayed slightly with the disturbed air as he swept the towel out from underneath it.

Marchbanks took the towel from him and shook it out, sending more dust spinning through the air, before lightly swiping it over the windowpane and the desk of controls. The Doctor thanked him graciously.

"That's the PA control," the secretary said in a businesslike voice, pointing at a temptingly red button and its associated few volume dials. The Doctor flipped him a brilliant smile before spinning the forest volume up as high as it would go. "And I think those ones control the lights."

Sherlock crossed to the desk to look at the controls the redhead had pointed out. The dust had resettled into clumps around the dials; he wiped it carefully away in order to read the labels and confirm that they separated the dome into four bands of lights under the fabric of the dome's surface. There was also a master switch.

He looked up at the Doctor, his finger on the switch, to find that the green eyes were alight and sparkling. Sherlock raised a lazy eyebrow inquisitively and received a nod in reply. He grinned, suddenly and sharply, and flicked the switch off, plunging the forest into darkness.

His own shadow, in the dull lights from the observation box, twitched convulsively. Sherlock wondered what the Vashta Nerada looked like when it was already dark – ordinary shadows couldn't exist without light at all. At the very least, it must be unnerving. Or perhaps they thrived in it.

The PA system hummed and crackled as the Doctor turned it on. "Vashta Nerada," he called, and his voice was different again, not the boyishly enthusiastic voice he had showed off so much in the TARDIS or the quiet, tense, dangerous one he had used on Sherlock when they had first discovered the shadows in the forest. This one was bold, authoritative, angry. It was a voice that made even Sherlock want to obey.

"I am the Doctor," it continued, and the Timelord's face in the dull yellow lights was hard, unforgiving. "And you will listen to me." His eyes flickered over to Sherlock; he took it as a cue to turn the lights back on, keeping them at the intensity that they had been at when he switched them off.

The forest floor was a pool of shadow; at the reinstatement of the lights the pool shifted and formed back into distinct shapes. "Four people have died," the Doctor stated. "Good people. That ends now."

He lifted his finger from the PA system, clicking it off and turning to Sherlock, pulling the sonic screwdriver out of an inside pocket of his jacket and tossing it to him. "Setting thirty-seven," he said softly. "That'll boost the lights. Try not to look into the forest while you're doing it. That goes for all of you," he directed to Marchbanks and Hannah. "Light bright enough to hurt the Vashta Nerada will hurt you, as well."

Sherlock caught the screwdriver, but didn't immediately use it. John had used to tease him sometimes about his use of dramatic timing and effect, but he'd been in enough standoffs with dangerous criminals to understand its completely practical usefulness. He looked at the screwdriver for a minute before finding a tiny dial on one side and setting the thing to setting thirty-seven. He nodded at the Doctor, who flicked the PA back on.

The shadows had begun to fluctuate, shifting around like the dappling of sunlight through trees on the ground, creeping up the walls. At the renewed sound of the Doctor's voice, the movements became more frantic, more hurried. "A part of your swarm has attached itself to a friend of mine," the Doctor continued, but broke off abruptly at the frenzy from the swarm below.

"I AM TALKING!" He shouted, and his voice was terrible – Sherlock could almost feel the floor trembling as though his words were a thunderstorm. He took that as a cue, spinning the dials on the lights as high as they would go and then switching on the sonic screwdriver on and pointing it at them, throwing a hand over his eyes and turning his face away in time to see Hannah and Marchbanks do the same.

The flood of light was visible even so, their shadows throwing themselves against the shelves of the opposite wall like the silhouettes left behind after chemical explosions. The noise came a moment after; a horrible screeching like millions of disturbed and angry bats. Then Sherlock's legs buckled and his heart froze and he collapsed forwards with a sharp cry, his arms shooting out at the last minute to catch him and hold him up against the desk of controls, his eyes squeezed shut against the light that had stopped being blinding when he had dropped the screwdriver.

"Bob!"

Small, dark hands closed around his shoulders and Hannah's face swam into his vision. Sherlock shrank away from her. "Get away," he gasped, shrugging his shoulders in order to try and shake her off. She stepped back, looking hurt, but a quick glance at her feet showed that she still only possessed her own shadow. He would have breathed a sigh of relief if he could have breathed at all.

Larger, square hands took his face between them and directed his head to look up; the Doctor was staring at him, his green eyes brimming with concern. "The shadow, Doctor," he gasped, practically leaning on the other man, registering Hannah's understanding intake of breath without hearing it. "It's in my heart."

It was as if a dark, freezing cold hand had reached up into his chest from his stomach and wrapped around his heart, squeezing just enough to render his legs useless and his head fuzzy. The Doctor dropped his face, letting him fall back against the desk, and bent to pick up the screwdriver and reinstate the blinding light of before. Someone must have turned the PA back on, because he could hear it humming before the Doctor almost screamed, "LET HIM GO!"

The screeching filled Sherlock's head until he thought it might explode, and then his mouth was opening without him wanting it to, and the words, "Let us go," were tumbling out of it. He gasped and spluttered. The Doctor didn't move, keeping the screwdriver pointed at the light controls, his eyes fixed on Sherlock's feet.

The shadow, Sherlock noticed absently, was no longer even pretending to be an extension of Sherlock's own form, and was instead puddled at his feet, lapping up his ankles, tendrils of darkness running up the legs of his trousers and stark against the purple fabric of his shirt, centered around his heart. He clutched at his chest, gasping, staring at the Doctor and seeing on the square face with its impressive chin a similar expression of shock to the one he could feel on his own.

"I can destroy you," the Doctor said, his voice low but still clearly audible over the PA system, his eyes still fixed on Sherlock's. "I can destroy every single one of you from inside this little box. You can only destroy one of us."

Sherlock felt himself grinning. "You care about your one so much," his mouth said without permission. It was a disconcerting feeling, an utter lack of control, as though his mind was completely separate from the rest of his body, and the more he fought against it the more helpless he became.

The Doctor swallowed. "I came into this room with so much mercy," he said, his voice the same low and authoritative rumble. "I was so prepared to find a way to help you. Now you've used up all of that goodwill. Let him go, and I might be persuaded to let you go."

The hand around his heart squeezed harder. Sherlock moaned pathetically, his body automatically curling inwards against the pain. The Doctor actually growled. He flicked the button on the screwdriver, played with it a few times; Sherlock could hear the tone of the buzzing changing. The light blazed. Sherlock screwed his eyes shut against the ferocity of it, beating against his eyelids, the shadows screaming inside the forest and echoing in his head.

"You can survive indefinitely without killing," the Doctor said through clenched teeth, his voice still echoing through the PA. "I will give you back this forest if you give him back to me. Give him back and I will almost forgive everybody else you've murdered. I'll give you this forest to survive as long as nobody else dies."

The light grew, and grew, and Sherlock's very brain was burning as his heart seemed to drown in shadow, and he wanted to scream but he had lost all control of his voice, he could feel the shadows climbing his legs and he almost wished they would just finish him the way the others had died instead of drawing it out like this, and the Doctor was still talking, something about knowing if they killed anyone else but Sherlock could see the bluff in his posture and the way he was moving his hands and he knew that the lights were as high as the Timelord could make them and if this wasn't killing them then there wasn't much more that they could do –

And then there were hands on Sherlock's face, gripping and shifting it, patting his cheeks gently as though he had passed out. He opened his eyes to find that he was looking up at the Doctor from the floor. The hand on his heart had gone.

"I'm all right," he said, managing to keep the note of surprise out of his voice and scrambling to his feet. He looked down, brushing his clothes off and seeing with relief that his feet were unobscured by shadow. "Where is it?" he asked.

The Doctor looked around quickly. "Off in the shadows somewhere, I should think," he said softly. "They accepted my terms and let you go."

Hannah breathed out shakily. "Will we still have to seal off the forest?" she asked, her voice trembling. Marchbanks put an arm around her, tugging her close, but his expression was curious.

"Probably not," the Doctor said, eyeing the display window looking onto the forest. "But I don't think you can be too careful. Best to keep everyone out. I'll give you the direct line to the TARDIS. If you ever have any problems with them again, call me."

Sherlock sat through the customary shaking-hands and thank-you speeches in irritation; the Doctor seemed to enjoy the praise, hugging Hannah and Marchbanks and even being relatively civil to Fitzherbert when he blustered out an apology. Sherlock withstood a hug from Hannah in patient silence, but he didn't quite breathe deeply again until the doors of the TARDIS clicked shut behind them and the Doctor sighed briskly.

"What will you do if they call you again?" Sherlock asked, following the alien up the stairs to the main console and sitting on the porch swing oddly placed there.

The Doctor flipped a control and grinned boyishly. "I'm sure we'll think of something." Before Sherlock could make a comment about being prepared, the alien had already moved on, leaning nonchalantly against the console with one finger resting on the wheel that Sherlock had already gathered controlled the time and place of their landing. "Where to next?" he asked, one eyebrow excitedly, almost flirtatiously quirked.

Sherlock grinned back. "Perhaps somewhere without the murderous flesh-eating aliens, this time," he suggested lightly.

The alien's green eyes narrowed. "Do dinosaurs count?"