"Home?" asks John. Hanna is long gone, Sally is using her gentlest manner to collect a statement from a very shaken-up Bryant, and they've both told the police everything they know, thrice. It's nearly dawn and John can hardly remember being so tired.

Sherlock shakes his head. "I've got a plane to catch."

"A plane? Now?! Where are you going?"

"I'll be back within the week. In accordance with our agreement."

Sherlock is used to wandering the world with relative freedom, but it gives John palpitations when he disappears for weeks on end and leaves no way to contact him or hint of where he's gone. They've reached an understanding in which Sherlock can go be mysterious, provided he gives John a time frame, takes his mobile, and provides some way for John to find where he's gone if he's not back on time. He agrees to answer if John calls, and John agrees not call unless it's an emergency. Technically, John could do the same, although it would never occur to him to take off without telling someone where he'd gone, and he doubts Sherlock would obey the rules if he tried.

John frowns, but doesn't object. Sherlock doesn't do well on a tether. "All right, if you must. I'll get Bryant home when they're done with him."

Sherlock nods. "I'll expect you to have something decent lined up to work on when I get back."

John glances to see if anyone is watching. He traces a finger affectionately along the line of Sherlock's lapel, and steps just a little closer to him. "Just don't get in the middle of any wars or accuse any world leaders of treason, okay?"

Sherlock claps him on the upper arm, grins, and disappears, leaving John shaking his head tolerantly.

The truth is he adores that Sherlock is wild and free like that, that he can't be kept or caged. He'd never try to take that away. He just wishes Sherlock weren't so secretive, that he didn't have to worry that something's happened when he's away. Then again, if Sherlock did tell John what he was doing half the time, John probably would refuse to ever let him go alone.

John finally arrives home mid-morning and collapses on the sofa. When he wakes, Mrs. Hudson brings him a hot meal, for which he is eternally grateful. Between the stun grenade and the fighting he's quite sore, and feels a bit aimless without a case. He eats heartily and takes a long soak, which revives him, but he's still not quite sure what to do with himself.

Usually, when Sherlock's away, he uses the time to detoxify the flat, get rid of old experiments in danger of becoming more serious health hazards, and generally prevent the place from drifting into condemnable territory. And it's also a good chance to spend some time with friends he doesn't see often.

He plans to do all those things, but realises he also wants to do something for Sherlock, something to surprise him with on his return. He's just not sure what.

They've been hard on each other lately. Neither of them is always good at being in a relationship. Though their devotion is absolute, they both have deep issues that don't just disappear with finding the right person. Things have been good since the Woman disappeared, but John wants to find a way to communicate to Sherlock that the rocky patch was a sign of them both growing, not coming apart, that John is still in it for the long haul.

It's not until the next day, after John's had some more sleep and cleaned three trays of decaying large intestine and half a dozen dead guinea pigs out of the refrigerator, that he knows what he should do.

They've merged bank accounts, but as about 80% of the money in it is from Sherlock, John still feels strange spending on anything other than household necessities. Still, Sherlock's got his trust fund and the only other option is to ask Mycroft, and since John never counts his part in cases when adding things up, he figures he's not out of line to spend a bit, particular if it's on Sherlock himself.

The project takes most of the week to complete, and John gets virtually no rest during the process, but thankfully it's done in time for Sherlock to return home six days later. He bursts in, completely unannounced as usual, and sinks into his chair.

"Ten hour layover in Bahrain, what kind of monsters run British Airways?" he complains.

John shakes his head sympathetically.

So, he was in the Middle East or North Africa somewhere. Interesting.

"Where's my computer?" Sherlock demands. "Any cases?"

"Oh, I think it's upstairs in my room," John tells him casually.

Sherlock scowls. "Why the devil would it be up there?"

"I borrowed it."

"Why would you do that?"

"Why do you borrow mine?"

Sherlock glares at him. "Well, go and fetch it."

"Ha! No." John walks into the kitchen and puts the kettle on, firmly ignoring him.

Heaving a great sigh of injustice, Sherlock gets out of his chair and pounds up the stairs, at loudly as possible. John waits about thirty seconds and follows him.

Sherlock is standing in the middle of the bedroom, surveying it in silent awe.

"Do you like it?" John asks, shyly.

The bed is gone. Two walls have been lined with book shelves, containing most of Sherlock's chemical library. One row of shelves is stocked with most of Sherlock's chemistry supplies. There's a rack for glassware, a sink, and hook-ups for gas, C02, and steam. The north side of the room is entirely taken up by large fume hood that vents outside. There is also a refrigerator and a chest freezer where John's bed used to be, as well as a steriliser.

It might be too much to think Sherlock might actually use that last one, but John can live in hope. At least it exists and something that could be used. If only, in fact, by John.

Sherlock doesn't say anything and appears basically frozen to the spot.

"I'll still keep some of my things up here in the cupboard," John says quickly. "I did move most of my clothes into your room… our room… but I tried not to mess up your system. I didn't want to clutter it up, I know you hate that, so anything I don't use a lot will stay up here. I hope you don't—"

"John." Sherlock cuts him off hoarsely. "John, what did you…"

John smiles a little nervously. "Well, you're always saying how it's wasteful to have two bedrooms and that you could use this space as a lab… I listened. Is it… okay?"

"Okay?" Sherlock says incredulously. He reaches out and grabs John, spinning him around. "John, it's…" He's grasping for words.

"Good," says John. "I'm glad."

Sherlock puts his forehead to John's. "You do realise that I'm still going to do experiments in the kitchen and leave body parts in the fridge and cultures laying about downstairs?"

Of course he will, because if nothing else Sherlock likes to be the centre of attention, he likes to be in the middle of things. But that wasn't the point of this exercise, and there's plenty of things Sherlock will use this for, particularly when he's in one of his more antisocial moods.

"Yes, I figured as much. But maybe anything potentially deadly can go up here? In the nice, safe hood?"

Sherlock nods enthusiastically. "Yes. Of course."

John puts a hand up and caresses the side of Sherlock's face. "We've been jealous and insecure and angry with each other," he whispers ferociously. "We're not perfect, we never will be. But I'm yours, and I'm not going anywhere. This is a promise. I'm sorry if keeping my room made you feel like I wasn't committed. I am and I always have been. Okay?"

Sherlock is silent, running his fingers through John's short hair. Finally, he says. "Yes." He kisses John, in an almost innocent way, and then takes a step back to look at him fully. He shakes off the emotional reaction to John's gift that he's been trying hide, and puts on a more devilish air. "Maybe we should break in my new lab properly," he says suggestively.

As always, Sherlock channels the sentimental into the sexual. It's the only way he cope without blowing a gasket, John suspects. He can't tell John how he feels, but he can show him. He can reward him with actions, if not words.

John laughs. "No, definitely not. How about we re-christen your bedroom as ours? Since, you know, there's a bed in there and not up here, anymore. That was rather where I was going with this whole thing."

Sherlock furrows his brow for moment and then brightens, grabbing John's hand and half dragging him downstairs.

Some months later, John is going through the remaining stuff in his cupboard. He's in a purging mood, and has decided that anything he hasn't looked at in the past six months, with the exception of photographs and army mementos, needs to go.

He's digging through a box that pretty much only contains terrible clothing gifts from Harry, going back to 1991, when he comes across an unlabelled envelope. He opens it and reads in Sherlock's writing, "Karachi, Pakistan".

John stares at it, confused, for a moment. Then it dawns on him. This must have been from one of Sherlock's trips, his note on where he'd gone in case things went wrong. How he'd expected John to find it in a box he hadn't gone through in several years, John has no idea, but it is rather consistent with Sherlock's thought process. But which trip had this been? He's only been gone a few times in the past year, since their agreement has been in force.

Suddenly, he remembers his conversation with Mycroft, weeks ago. Irene Adler had been killed by a terrorist cell in Karachi. He hadn't had the heart to tell Sherlock. But now it seems like he hadn't needed to. He finds a mix of anger and empathy swelling within him.

He sits carefully, waiting until he can think rationally about this. Once he's worked it through, he gets up calmly and goes downstairs. Sherlock is in the bedroom, resting after a particular convoluted and energetic case of drug smuggling. The time spent in the meat locker had been particularly exhausting for them both.

John walks into the bedroom quietly. Sherlock is lounging on the bed in pyjamas bottoms and an inside out t-shirt, eyes closed but not asleep.

"Hey," John says neutrally, climbing on to the bed. "Can I talk to you for a minute?"

"Can I stop you?" Sherlock replies, not opening his eyes.

"Probably not," John admits. He flops down on his stomach next to Sherlock and props himself up on his elbows. He says, keeping frustration and hurt out of his voice as far possible, "Why didn't you tell me you went to rescue Irene Adler?"

Sherlock's face twitches, but manages not to betray any strong reaction. "Why did you tell me she was in a witness protection scheme in America when you knew she was dead?"

John feels a pang of guilt for a second, unless he remembers that he only lied because he was afraid if Sherlock knew she was dead, he might just self-destruct.

"Well, she's not dead, is she?" John retorts, sharper than he means to. He sighs, and sits up, taking Sherlock's hand. "Sorry. I just feel foolish now. I'm not angry, not really. I'm glad she didn't die, she didn't deserve that ending. I just want to understand. That's all I've ever wanted."

Sherlock finally opens his eyes and gets up reluctantly, facing John cross-legged. "I'm not sure I can explain."

"Try," John says firmly. "I'm not jealous anymore. But I want to know you as much as I can, and this is a part of you I just don't know. I won't react badly, whatever you say. Just…please try?"

Sherlock nods reluctantly and takes a deep breath. "I know you think I'm obsessed, infatuated. You're right, I am. Have been. It's not because… it's not an attraction. Not like you would think. I am…drawn to her. Because… John, this is embarrassing!" he says suddenly.

"There's nothing to be embarrassed about," John assures him. "Whatever you feel, you feel, I just want to know. Isn't that what this is all about? Knowing each other?"

"Fine," Sherlock agrees grumpily. "The fascination is… that… she is… me…"

"She's you? No she's not!" John exclaims. "You couldn't be more unlike. Well, except that you both are a bit heartless and possess a strange amorality that few people share."

Even as John protests, he can see it. She's crueller than Sherlock, untempered by any affection, and she prefers the mystery of the man to the mystery of the crime scene, but there is something there, some spark they've recognized in each other.

Sherlock makes a noise of frustration. "But we are the same. And yet not. Or at least… she's who I might have been. If I'd been born different, if I'd been female. She's just as clever. It's merely a different kind. I'm clever about facts, she's clever about people. She can read a person, a social situation the way I read a crime scene. She's like an alternate history to myself. All the skills I lack, she possesses."

John takes that in. "So, you're saying she's…a different version of you?"

"Yes. No. I don't know." Sherlock is breathing too fast, unable to put it in to words, nervous that John will end up mad at him again.

"Okay," John tells him, rubbing Sherlock's arm reassuringly. "It's okay. Just tell me what you can."

Sherlock licks his lips. "I need her to exist. That's why I saved her. She's like… my equal and opposite force. She's the balance to me in the universe, what might have been. Not like Moriarty. He's what I could become but mustn't. And not like you. You're my complement, my other half."

The Woman, the Villian, and John. What will never be, what could be, what is now. These three people tell Sherlock who he is. And who he's not.

Sherlock continues, "You, I need to be a person, to make me whole. Her, I need as a counterweight, to stand in relief. She's my negative polarity. Or maybe I'm hers. We orbit each other, in each other's influence but if we get too close everything burns. As long as she exists, I can exist. If there's not hope for her… I think there might not be any hope for me."

"You… need me in order to be a person?" John asks, stunned.

"That… wasn't really my point," Sherlock says. "But, yes, if you need to know. I should have thought it was obvious."

"Sherlock Holmes, you are the most breathtaking moron I've ever encountered," John tells him. He climbs into in Sherlock's lap, winding his legs around Sherlock's waist and hooking his arms around the back of his neck. "But then I'm not much better."

Sherlock looks confused. "You understand then?

"Not really. I've never had something like that, I'm not sure I can ever really understand it. But I can understand what it means to you. And I understand what a fool I was to think that you would let her come between us. I'm sorry."

He leans forward and nuzzles the hair above Sherlock's ear in atonement. Sherlock exhales slowly. "I… regret… if any of the ways I have behaved made you doubt your importance to me. I was distracted. I didn't ever expect to meet someone like her. Like myself, and unlike. Trying to puzzle it out took more of my attention than perhaps it should have."

John sometimes aches to see Sherlock struggle with concepts and feelings most people understand so well they hardly need to explain them. He has to construct metaphors and invent terms just to be able to grasp what's going on within himself. Everything that has to do with people, with their emotions and his, and their relation to each other, is a battle for him. No wonder he finds refuge in the coldly logical so much of the time.

"Apology accepted," John whispers.

"Likewise." Sherlock is trembling in his arms, from the energetic cost of an explanation like this.

John holds him tighter. "I was just so afraid of losing you. I went a bit barmy. What can I do to make it up to you?"

Sherlock puts his mouth to John's ear and tells him very softly, almost bashfully.

John throws his head back and laughs, then kisses him deeply, passionately. "Why, Mr. Holmes, that could take all afternoon!" he says, with feigned shock.

Sherlock seems puzzled. "If you don't—"

John slaps a finger across his mouth. "I just meant that we'd better get started."

He rolls himself to the side, pulling Sherlock down to the mattress with him. "You're my dark matter," he tells Sherlock, stroking the curve of his hip. "You can't always see it, but you make up ninety-six percent of my universe."

"You look like a beacon in the North Sea," Sherlock replies, fervently, glad to be speaking a language he understands again.

John smiles and draws Sherlock nearer, and suddenly neither of them are thinking of Irene Adler, nor will they for a very long time.

This series will be continued in Part 5, "Invisible Spectrum".