As the boat drifted further from the City, the smoke from the hotel still visible on the skyline, the Mallrats returned one by one to the small cabin, leaving their home behind. The low purr of the vessel's engine cut out the noise from outside, but had anybody been out on deck they might have caught a faint whooshing sound from somewhere in the City, Sector 10 to be precise, not far from the Phoenix Mall. Just as the Mallrats were leaving their home, someone else was arriving…

If the City had not been evacuated, a passerby might have seen a blue box materialise on the street corner, a door on the side opening to give a glimpse of an inside that was bigger than the outside, but in a world where Reality Space and holograms had, at least until very recently, been a fact of life, the sight would have been just another oddity among many. The two figures who emerged from the box, however, were something else entirely. A man dashed out first in a long brown coat over a blue suit, flinging his arms out wide and taking a deep breath of the air as if savouring it, a huge grin on his face, which was bare of tribal markings. The same was true of the woman who followed him out, smartly dressed in a black jacket and trousers, with long auburn hair and carrying a helium balloon that read 'Happy Birthday!'

A man and a woman. Two adults, in a world where all the adults were dead.

'Here we are, Donna!' the man said, sounding pleased with himself. 'Home Sweet…'

He paused, still with his arms outflung, scanning the city before him with confused eyes.

'Hang on a minute, that's not right. Where are we?' he pouted, scratching his head. He turned to the woman, Donna, with a look reminiscent of a guilty child.

She gave him a distinctly unimpressed look, arching one eyebrow. 'Well it's not Chiswick, I'll tell you that!' she said flatly, before surveying the rubbish-strewn roads and graffiti-laden walls with contempt. 'What kind of run-down council estate have you brought me to? It's probably right on the other side of town as well, we might as well be on the Moon!'

'It's no problem!' the man said defensively, once he could get a word in. 'We must have just taken a wrong turning, that's all, a few adjustments and we'll be right on course again. Wait here.'

'I don't know,' Donna sighed as the man ran back into the box. 'You men are all the same aren't you?' She allowed herself a small smile, speaking now in the familiar way of two friends poking gentle fun at one another. 'No matter which planet you're from, you won't stop and ask for directions. I'm travelling by time machine and I'm still going to be late for Gramps' party! How embarrassing is that?'

'Hey, I wasn't that far out!' the man answered, sounding more distant than the size of the box seemed to allow. 'Right planet, right dimension, just a teensy bit further into the future than I was aiming for.' His voice seemed to come closer until he emerged from the box once more, holding his thumb and forefinger a couple of centimetres apart. 'See?' he said, pointing to the measurement he was making. 'Teeny tiny! Now, remind me to take a look at the brakes when we get back, will you?'

Donna watched him lock the box and march out onto the street with a puzzled look on her face. 'Get back from where? Doctor?'

The Doctor turned around. 'Don't you want to have a look around?' he asked. 'Find out where and when we are?' A glint appeared in his eye at the prospect of exploring this strange new world.

'We're already late…' she began.

'Donna,' he said simply, then paused, as if waiting for her to grasp something.

She just stared at him blankly. He leaned forward slightly, as if willing the penny to drop.

'What?' she said, in an unbothered tone.

The Doctor waved his arms at the box behind her. Seconds passed.

'We've got a time machine!' he spluttered finally, defeated.

'Oh. Oh, yes!' A similar glint of anticipation appeared in her eyes and she giggled excitedly. 'Well, lead on then! Wait! What am I going to do with this?' She pointed up to where the balloon bobbed above her head.

The Doctor took the balloon from her hand and tied it in a bow around her wrist. 'Beautiful!' he said, stepping back to admire his handiwork. Donna tutted, but stepped forward and linked arms with him.

'Come on then,' she said, mock-impatiently.

They walked through the city streets, cautiously at first, not knowing who or what they might find, but the further they walked the more they came to realise that the City had been abandoned. Every street was strewn with rubbish and graffiti covered everything – the walls, the windows, the vandalised cars lining every road, even some of the larger pieces of rubbish. Clearly order had been lost here a long time ago, but now it seemed even the vandals and litterbugs had left this place to its fate. Apart from the occasional tin can rolling along the road, the only sounds were the Doctor and Donna's footsteps, which echoed oppressively.

'I don't like this Doctor,' Donna said nervously. 'Where is everyone?'

'Good question,' the Doctor replied half to himself, his brow creased in thought. 'It's like the entire City just up and left. And recently too, look at that.'

Donna walked over to where the Doctor was pointing. An upturned barrel stood on the street corner, the remains of a card game on top, and a bottle of water lying on its side as if knocked over when the players had left. The tarmac of the road below was darker there, still wet from the spilled water.

'Maybe…maybe everyone's gone inside?' Donna voiced. 'Maybe there's a curfew?'

'Oh now, that is clever Donna Noble!' the Doctor smiled, wagging a finger. 'Let's go inside and find out!'

'Next time I have a good idea,' Donna grimaced as the Doctor grabbed her hand and rushed to the nearest building. 'Remind me to keep my mouth shut.' The words 'Central City Plaza' stood out above the doors as they passed through into darkness.

'Hello?' the Doctor called once they were inside. 'Anyone home?' There was no reply.

'It's a shopping mall!' Donna exclaimed as they entered. Having feared what they might find inside, she was suddenly buoyed by the normality of the surroundings. 'Hey, if there's nobody here we should have a look around. You could do with a new outfit.' She headed further into the mall, exploring.

'What's wrong with my outfit?' the Doctor sulked.

'All I'm saying is it wouldn't hurt to have a change every now and then,' she argued, popping her head into a few shops. 'That's strange,' she said.

'What's that?'

'Well you're right, it looks like this place was recently deserted, but the building itself hasn't been used as a mall for longer than that. It looks like people have been living here. Do you think there's been some sort of natural disaster? Maybe this was converted into accommodation for the homeless. But…I don't know, it all seems too…organised, for that.'

The Doctor narrowed his eyes, thinking.

'Doctor I know that look,' Donna said. 'It usually means trouble. Do you think whatever disaster there was is going to happen again? Is that why there's no people here? Have they already been evacuated?'

'Where are we?' the Doctor mused. 'Something about all this…seems so familiar. If I only knew which city…'

Donna rounded the corner into the main courtyard of the precinct. A large fountain, dry but still impressive, in the form of a phoenix, loomed up at her. Behind it two sets of stairs joined and led up to higher floors. The piece of wall between the stairs was covered in graffiti like those outside.

'Mallrats,' she read. 'Vandals, more like. Kids who don't know how to respect…'

'What did you say?' the Doctor asked, rushing to join her. His face paled when he saw the fountain, and the graffiti. 'Oh no. No, no, no, it can't be.'

'What is it? Doctor?'

The Doctor ran up the stairs, taking them two at a time. Donna followed him, all the way to the roof. The door burst open at the top and the Doctor ran to the edge, looking out to the horizon where the sea was clearly visible, glinting in the sunlight. They were somewhere on the coast then, not that that helped much. She pulled the balloon further down towards her so the wind wouldn't catch it as much.

'There!' the Doctor said, pointing out to sea. Donna looked in that direction but couldn't see whatever it was the Doctor was showing her. 'The boat,' he added. She squinted. She could just about make out a small speck that could have been a boat, moving away from the City.

'Who is it?' she asked. 'Do you know where we are?'

'No idea,' he said with a mirthless laugh. 'But there's your Mallrats, escaping while they can.'

'I don't understand,' Donna said. 'Are we in danger?' The Doctor just carried on looking out to sea. 'Wait, Doctor, can you smell smoke?' She sniffed the air, then scanned the skyline. 'Yes, it is! Over there, look! That building is on fire! There ARE people here!'

'The Doctor looked over his shoulder to where Donna was pointing. Sure enough, one of the buildings there was ablaze, its thick black smoke belching out into the air.

'Back to the TARDIS, Donna,' he said quietly, but firmly. 'Now.'

'What's going on?' Donna asked, becoming increasingly nervous. 'You know something! Tell me!' she grabbed his arm as they reached the stairs back down to the mall, but he didn't stop.

'I'll tell you when we get back to the TARDIS,' he said urgently, but in a reassuring rather than an angry tone. 'Just trust me.' The two of them ran down the flights of stairs, Donna's balloon bumping against the roof as she ran. Past the broken fountain they ran, and back out onto the street.

Two figures were standing right in front of the door. Donna screamed.

The two figures, the two children, clutched each other and took a couple of steps backwards, just as surprised as Donna had been. They were both young girls, the oldest probably around twelve and the youngest no older than seven, Donna estimated, though on closer inspection they both looked ancient. Both children's hair was grey, and their faces were beginning to show signs of wrinkles. Their clothes were a mismatch of several different colours and styles, quite jarring to the eye, and what looked like the remains of war paint marked their faces. What kind of Earth had the Doctor brought her too? At first the city, the buildings…it had all seemed so…present. But this, this was something different entirely.

The younger child started coughing, an awful hacking wheeze, and Donna saw that there were red blotches on her skin.

'Oh, you poor thing,' she whispered, reaching out to her. The little girl seemed surprised at first, but reached out too, as if eager to be touched. Their hands brushed, then she felt herself being dragged backwards. By the Doctor.

'Donna, no!' he cried. 'We have to leave. Now.'

'But Doctor, look at her! Surely you can do something to help her?'

'I wish I could,' he said sadly. 'I really do. But my main concern right now is your safety. We have to get back to the TARDIS.'

'She's just a child!' Donna protested angrily as the Doctor pulled her away, giving the two children as wide a berth as he could. 'Doctor!' Their disappointed faces haunted her as she followed, held by his firm grip on her arm. The Doctor refused to look at them, but she couldn't tear her eyes away, no matter how much she wanted to.

'STOP!' Donna yelled, a few streets down from the mall and those poor children, her words coming between anguished sobs. 'I'm not going another step until you tell me what's going on! You know, Doctor! I know you do! Tell me!'

'And I will,' he said in a pleading tone, keeping his eyes on the street ahead and not slowing his pace. 'Just please…' he turned to face her and stopped dead. She stumbled ahead a few steps before stopping also. Her hair fell in front of her face and she raised a hand to move it away. Her hair. Her white hair.

'What's happening to me?' she gasped. 'Doctor! What's happening to me?'

'So fast,' he whispered. 'Oh Donna…Oh Donna I'm so sorry, I don't…I don't know what to do.'

'Doctor,' she repeated, slowly and more calmly than she felt. 'What. Is happening. To me?'

'It's a virus,' he explained, holding her arms. There was a pained expression on his face that wasn't very soothing. 'The human race, it got so caught up in its quest for beauty and youth…there was an experiment, an anti-ageing product. Something went wrong and the Virus was released. It caused rapid ageing and sickness, and wiped out the whole of the adult population. On the whole of the planet Earth, only the children survived, forming tribes to stay alive…'

'Doctor, no offence,' Donna coughed, but by the time you finish this story, I'm going to be an old lady myself.'

The Doctor laughed sadly. 'Oh Donna, it was the Mallrats…they were only children themselves but they pulled the City together in the aftermath. Only…only it happened again, and this place, this City…there was no antidote this time, it just…died.'

'Like I will?' she asked quietly. 'And those children?' Her eyes pleaded with the Doctor to give her good news, but he said nothing, barely meeting her eyes. 'But Mum, and Gramps…they're waiting for me. At the party.' She put a hand to her mouth as she realised the Doctor would have to break the news to them in the middle of the celebrations. She knew she was being irrational, clinging to that part of it, but she didn't want to ruin the party.

'No,' the Doctor said suddenly. Firmly. 'I won't let you die. There must be some way to find a cure…' She was startled to be picked up and carried as the Doctor ran back the way they had come, further into the City.

'Where are we going?' she asked.

'The one place where there might be answers,' he replied without breaking stride. She flung her arms around his neck to steady herself. The Doctor carried her a few blocks before putting her down and looking around anxiously, clutching his hands in his hair. 'No,' he muttered angrily to himself. 'Too slow, too slow…Think, think, think, have to get there quickly…Yes! Wait there Donna, I'll be back!' He ran forwards a few steps then turned back to look at her. 'I promise,' he added gently, then he was off again, disappearing around a corner and leaving Donna standing alone in the deserted street. Trusting that he would return, she went to sit on the kerb to wait for him, coughing as she did so; it didn't feel right sitting in the middle of the road, even if this place was deserted. Earth, the Doctor had said, and not too far into her future. How long? Looking carefully at the buildings and vehicles around her, looking through the marks of anarchy and children running rampage, none of it would have looked out of place in her own time, in London. She fiddled with the balloon ribbon around her wrist sadly, not wanting to look at the city anymore. Her hands were wrinkled and marked with age spots. If she concentrated, she could almost see her skin shifting, feel it itch as it changed at an accelerated rate. The Doctor would probably have a fancy word for it.

'Look! There's another one!'

The voice jolted her out of her thoughts and she got to her feet, just as a group of youths crossed the street warily towards her, coming to stand in a semicircle around her, but a few metres away, not wanting to get too close. They were all dressed in clothes of a variety of styles and colours, just like the two girls outside the mall had been, and each one held a dust mask to his or her face. There were seven of them in total, four boys and three girls.

'What do we do with her, Dane?' one of the boys spoke through his mask. He wore a spiked choker around his neck, and his dark blue Mohawk made him look taller than he was. She could tell that he was smirking behind the mask and instantly disliked him.

Dane, the boy directly in front of her and evidently the leader of the group, looked at him. 'The same thing we did with the others, Isaac,' he replied. 'But you don't have to look like you're enjoying it so much.' He turned to face her again, piercing brown eyes framed by the silver paint surrounding them and covering his forehead, beneath short-cut, black spiked hair. He raised a hand and she gasped in terror as she saw the brick he held and realised the group's intention. The smarmy boy with the Mohawk, Issac, had stepped forward too, softly tapping a baseball bat into the palm of his hand.

'No, please!' she cried, raising her hands to shield herself and wishing desperately that the balloon would carry her away. 'You don't have to do this; my friend, the Doctor, he'll be here soon, and…'

'It has to be done,' Dane said, interrupting her. 'It's all about survival, you should know that by now. Or has the Virus taken your mind already? We have to contain it, or we'll all be dead.'

'What tribe are you from?' one of the girls asked, joining Dane and Isaac. Her pink hair and make-up seemed at odds with the army-style camouflage trousers and combat boots she wore. 'I don't see any markings on you.'

'You're all mad!' Donna yelled. 'Leave me alone!'

'Answer her!' Isaac snarled, raising his bat.

'I don't know what you're talking about!' she sobbed. 'Please!'

'Don't play dumb with us!' the angry kid shouted, moving towards her.

'I don't…wait, Mallrats! Mallrats!' Donna stammered, suddenly remembering the group of children the Doctor had spoken of. It seemed the only name that fit with what the girl was asking, the only thing that might give her a reprieve from Isaac's zeal.

'Stop!' one of the other boys shouted at Isaac, who whirled around angrily to face him. The boy had his arm around a frightened looking girl of about fifteen, with long black hair that fell messily around her shoulders. 'The Mallrats rescued my sister from the Technos' Reality Space labs. Maybe we could just let this one go?' He smiled at the girl, presumably his sister, and she gave a timid smile in return.

'He's right,' the girl with the pink hair agreed. 'None of us would be here if it weren't for the Mallrats. They were the ones who found the antidote. They stood up to the Chosen. They brought down Mega!'

'And look what that's brought us!' Isaac argued, with a gesture that took in the empty streets and Donna. 'She deserves all she gets!'

'The Mallrats did a lot for the City, it's true,' Dane agreed. 'But that doesn't change anything. I'm sorry, but we agreed. It's the only way to stop the Virus spreading.' Some of the group looked uncomfortable, but none of them spoke against him.

'Please,' Donna whispered again. 'Don't hurt me.' But she knew it was too late as Isaac gleefully raised his bat once more, followed by Dane.

Suddenly the sound of a siren made everyone jump, and looks of pure terror appeared on the children's faces. The brick Dane was holding fell to the ground with a thud, and Donna grimaced as she tried not to imagine the sound it would have made slamming into her skull.

'Zoot!' the frightened girl cried. She, her brother and another of the boys ran away as fast as they could.

'Let's get out of here,' the girl with the pink hair urged Dane, pulling on his arm. He started to follow but Isaac stopped him.

'Zoot's dead!' the teenager spat. 'It's just some kids messing around!'

The siren sounded closer now, and Donna couldn't believe her eyes when a police car appeared from around the corner, two red flags attached to the sides. The windows were all blacked out, and she got the horrible impression that nobody was driving it at all. It drove towards them.

'Do you really want to stay around to find out?' Dane snapped at Isaac, before leaving at a brisk pace with the remaining girls. Isaac took one look at Donna, then gave a frustrated grunt and followed them. The police car stopped right where they had been. The siren stopped and there was a scrabbling from inside before the Doctor emerged from the sunroof.

'Get in,' he urged, fumbling for his sonic screwdriver in his overcoat pocket. He pointed it at the back door on Donna's side, which flew open, and she scrambled in, pulling the balloon in with her, relieved to be off the streets.

'Am I glad to see you!' she gushed.

The Doctor climbed back down into the driver's seat and looked at her. 'Are you alright?' he asked.

'Kids these days,' she noted. 'Don't know how to respect their elders.'

'That's my Donna,' the Doctor smiled, then reached out to touch her face. 'But this, this is all wrong.'

'Don't hold back, Doctor,' she quipped. 'We can't all look as good for our age as you.'

'No, I mean this shouldn't be happening,' he said. 'Not here, not in this time. This is beyond anything human technology could create. Which means something else is here.'

'Something alien?'

'Something alien,' he agreed. 'Which means I can stop it. I'm going to save you, Donna.'

'Then let's go!' she said. 'I'm not getting any younger!'

The Doctor did something with his sonic screwdriver and the car sped off into the city. It soon became apparent that he was taking her towards the burning building she had seen from the roof of the mall. As they approached, Donna could see that it was, or at least had been, a hotel. Only a shell remained of the façade now that the fire had burned itself out, but a fallen, charred sign discovered upon stepping out of the car announced that in more recent times it had been Ram's Palace, and somebody else's before that. Ebony, she thought the name was. Both children, if she understood what she had seen and what the Doctor had told her correctly. What kind of place must this have been? Were gangs of children with weapons – tribes, the Doctor had said – normal, or was that just a result of what had happened to make everyone leave? The Doctor took her by the hand and led her into the remains of the building. The walls were mostly blackened from the fire, but from what she could make out it looked as though this building had been better maintained than other parts of the City. As they moved further into the building, picking their way carefully over the rubble, it became clear that the far end of the building, past the old hotel swimming pool, had survived better than the front.

'Long live the Human class system,' the Doctor noted wryly. 'It looks like there was an explosion in the generator room that damaged most of the hotel, but the executive suites were placed far enough away from the noisy inner workings of the building to have got off lightly. If there's anything of use in this place, that's where it will be. How are you feeling? Any more symptoms?'

'I'm fine,' she lied, and her voice gave her away. It sounded…wrong; it was thin and reedy, and from what she could see of her hands her skin continued to age rapidly, if not as quickly as her hair had changed colour. At least there were none of those red blotches that the children had had. Yet. The Doctor quickened his already brisk steps.

All of a sudden they reached an area of corridor that was completely free from signs of the fire that had gutted other parts of the building. At one point the walls were sooty, the next unmarked, then further down the corridor they became sooty again. Within that clear stretch, a single door was set into the wall. The Doctor walked straight through and Donna followed. Inside, the room looked more like a control centre than a hotel room. Monitors covered the walls and there were desks holding laptops. There was also a boy in glasses lying slumped on a chair in the middle of the room, with what looked like a virtual reality headset dangling from his hand. He wore a strange uniform of black and red, with a car seatbelt around his waist and a black T painted on his forehead inside two red circles. The Doctor rushed over to him straight away, but after only a moment's inspection shook his head. He was dead.

'CONSCIOUSNESS DETERMINING EXIS…EXISTENCE…' a voice rattled from somewhere in the room. Donna turned to see the face of a boy wearing a peaked hat and goggles appear on a computer screen. 'INTRUDERS!' it announced, before the computer sparked and died. She was sure she heard a tinny scream of agony from the voice.

'Quiet,' the Doctor said softly, only half paying attention. He was still looking sadly at the dead boy, but was lowering his sonic screwdriver from where he had pointed it at the talking computer. 'We won't be having any of that.'

'What…what was that?' Donna asked, pointing at it with an alarmingly bony hand.

'Artificial Intelligence,' the Doctor replied. 'Not much was written down about this time – what is it with kids and books? Don't they know knowledge is power? You can find anything in books if you know where to look. For instance, I've read all there is to read about the time of the Virus and one of the few things they all agree on is that all this, the Second Wave and the Exodus of the Mallrats, it was all caused by that thing.' He pointed accusingly at the now silent computer.

Donna frowned in thought. Her head felt like cotton wool, but she was sure she remembered…yes! 'Those kids that tried to attack me,' she said. 'One of them talked about Technos. Is this what he meant? All these computers and stuff?'

'It must be,' the Doctor agreed. 'The legends say that the Technos were the scourge of the City in the last days. You wouldn't think it to look at him would you?' He traced the painted T on the dead boy's forehead with his finger.

'Him?' Donna gasped. 'But he's just a boy.'

'Indeed. Which makes you wonder doesn't it?' the Doctor mused.

'It does?'

'All this Artificial Intelligence and Virtual Reality; how many children do you know who could create all that?'

'Mum's always getting Mrs Baxter's kids down the road to fix her computer,' Donna said thoughtfully. 'And Danny Jeffries from Gramps' astronomy club's only fifteen but he had detectives round his house a couple of months ago and everyone said he'd been hacking into NASA.' The Doctor gave her a flat look. 'But I suppose this is on a slightly larger scale,' she admitted.

'And not forgetting,' the Doctor added, still staring at where his finger touched the boy's forehead. 'That something protected this room from the fire. Now I bet Danny Jeffries couldn't do that, could he? Not with simple Earth technology.'

'Very clever, Doctor,' the dead boy said, opening his eyes.

'There you are,' said the Doctor, calmly folding his arms.

Donna gasped in shock. 'Is he…alien?'

The Doctor shook his head. 'No,' he said. 'Not him, something inside him. Who are you? How do you know my name?' The boy just laughed. 'I command you to identify your place of origin and…'

'Oh come now, Doctor,' the boy interrupted. 'There's no need to start quoting the Shadow Proclamation. One of my kind can always sense one of yours, and you are the last of the Time Lords, are you not?' He sniffed the air and licked his lips. 'You have…such great age.'

'Tithonian!' the Doctor spat.

'What is it, Doctor?' Donna asked.

'Parasites!' he grimaced. 'Tithonians are intergalactic parasites that feed on Power and Chaos, but they're such lowly, pathetic creatures that even the Trickster won't use them. Their bodies are so unstable that they have to constantly feed to hold together a corporeal form. I never thought to find one here; most of them took to scavenging the Time War, too greedy to realise they'd be trapped inside the time-lock.'

'A feast indeed,' the Tithonian smirked. 'But some of us escaped.'

'That's impossible,' the Doctor frowned.

The Tithonian laughed again. 'There are ways, Doctor,' it boasted. 'It was not easy, and not pleasant, but it appears there are advantages to being lowly, pathetic creatures such as ourselves. A few hundred of us, perhaps, survived, and found ourselves on this primitive planet, with its inhabitants so absorbed by their own image, not even aware of the War that raged across the universe.'

'And I suppose it's just a coincidence that you arrived just in time for one of the greatest catastrophes to befall the Human race?'

'The Virus was not of our making,' the Tithonian said, then suddenly smirked. 'Originally. But we were starving, and our passage through the time-lock left us with even less control over our physical bodies. This new form was not without its advantages, however. We found that we could merge with the feeble minds of these humans, and use their bodies as our own. The benefits were apparent. With these host bodies we could use our knowledge to alter the structure of the ageing Virus, to greatly accelerate its reproduction in the body. Age was always a particular delicacy of ours, Doctor; the power of life itself, and the chaos of a life lived, the myriad choices and possible futures it could give birth to. We fed well in those days, while the stock lasted. Since then, we have wandered the Earth, feasting on the chaos left behind. I, Mega, am the only one to have remained on these shores. But the chaos is ending, Doctor. These younglings are creating order again! I had to stop them. It took so long to amass the technology in this cursed place, but the Virus spreads again!'

'And as we speak,' the Doctor said proudly. 'The Mallrats, those bothersome, order-creating younglings, are escaping across the sea; a journey that will see them lay the foundations for the Golden Age of Humanity. You see, that's the amazing thing about the Human race; no matter how many times you knock them down, they come back fighting!'

As if on cue, Donna collapsed, her wizened frame reaching out to the Doctor as she fell.

'Donna! No!' the Doctor cried out in anguish as he caught her. 'What next, Mega?' he shouted angrily. 'When the Humans are all dead? This planet can't sustain you, you'll die! All of you! You've broken galactic law coming here, feeding on a Level Five planet. Send out an antidote, reverse the Virus, or I'll alert the authorities.'

Mega rose, flexing lifeless limbs in the process. 'Tell the Shadow Proclamation if you wish,' he said. 'I will take my chances. This body has been useful, though the boy grew too strong for my control in the end, but the life has gone from it, and I must find a new host to feed on. Your companion will do nicely, for the little time she has left, but that will be enough to sustain my body. She is the oldest thing to have walked this planet in years! Apart from you, of course, Doctor. A pity this Virus cannot be tuned to Time Lords. I would have been all but immortal with your energy.' With a shudder the Tithonian threw back his arms and a pale mist oozed out of the host boy's skin, his body falling to the floor when the last of it had been expelled. The mist coalesced into a shape that was all tentacles, but translucent and insubstantial, as if only the memory of a body, sometimes beginning to fall away before snapping back into position as the parasite struggled to maintain a hold on its form.

'Stop!' the Doctor commanded. 'I can get you off this planet, take you somewhere you can survive. Just leave her be!'

'It's too late for that, Doctor,' Mega announced, its tentacles whipping in and out of shape with increasing ferocity. 'I need to feed now!'

'Then you leave me no choice,' the Doctor said coldly. 'One pulse from my TARDIS will have the atmosphere so charged that it'll pull the atoms from that sad excuse for a body you have. You and all your brethren will be sucked into the air!'

Mega laughed. 'And the Mallrats? The future of Humanity?'

The Doctor paused; the Tithonian was right. That much atmospheric excitation would have repercussions. Storms. Could he really risk tipping the Mallrats overboard? Changing the course of Human history to avenge Donna?

'Stand aside, Doctor. She is mine!'

He looked at her sadly. She was completely changed now, her wrinkled face framing eyes that no longer recognised him, red-black blotches covering her skin, and her frail wrist still holding the balloon he had tied there. Then he smiled, a grin so wide it split his face in two.

'Can I just say a few last words on Donna's behalf?' he asked jovially. 'She can't say them herself, poor old dear. But I can tell that what she's dying to say…oh, bad choice of words, sorry Donna.' He patted her hand. 'What she'd really like to say, is isn't it funny how helium makes your voice all squeaky? No?'

'Enough!' Mega snorted, stretching its tentacles out towards Donna. 'Oh, the savour! It has been so very long!'

'Of course what's more interesting,' the Doctor said, more seriously. 'Is that helium is an incredibly stable element, that can diffuse through a solid at three times the rate of air, which begs the question, what would happen if it passed through you?' In a flash his sonic screwdriver was in his hands, pointing at the balloon hovering above Donna's head. He chose the setting and pressed. The balloon expanded as the helium was excited, the gas pressing against the sides until it finally burst. The Tithonian was knocked back as if on a breeze, and wisps of tentacles began to break off as the helium passed through its body.

'Noooo! Doctor, help me!' Mega cried. 'Help me! Doctoooooor!'

'You left me no choice,' the Doctor said, as the last wisps faded into the air. The hairs on the back of his neck stood on end as forks of orange lightning suddenly streaked through the room, radiating from the spot where the alien had dissipated; the unbound life force of the parasite. Sparks hit both the Doctor and Donna.

'Ouch!' the Doctor exclaimed, pouting as he shook his hand where the lightning had hit. Next to him, Donna lay bathed in an orange light. As the glow subsided he smiled and helped her to her feet. She steadied herself for a moment, holding a hand to her head, then pushed her auburn hair back to look at him. Then she punched him in the arm.

'Poor old dear?' she complained. 'Hey! What are you smiling for?'

He grinned at her. 'Got my old Donna back!' he replied. 'Well, young Donna. Well…'

'Oi!'

They laughed, and embraced. 'It's good to have you back,' he said seriously.

A scrabbling noise behind them tore them apart, and they watched in shock as the previously dead boy who had been Mega's host got to his feet shakily.

'Doctor, what's happening?' Donna whispered. 'He was dead!'

'He must have been hit by the Tithonian's sparks just as we were,' the Doctor replied, then grinned. 'He must have been holding on, that tiniest spark of life left in him. Oh, what did I say Donna? You Humans, little fighters the lot of you!'

The boy suddenly became aware of their presence and stumbled back, wide-eyed. 'Who…who are you?' he stammered.

'It's ok,' the Doctor reassured him. 'I'm the Doctor, and this is Donna. We're friends. What's your name?'

'J-Josh,' the boy replied. 'My name's Josh.' He wet his lips nervously. 'You're adults?'

'No time to explain, Josh,' the Doctor said. 'Tell me, all this,' he gestured to the computer equipment that filled the room. 'How much was you and how much was Mega? Can you remember how to use it?'

Josh put a hand to his head, still trying to get to grips with what was happening to him. 'I think so,' he finally said.

'Good, because I need you to operate the communications system. I'm assuming you have one,' the Doctor barely waited for the boy's nod before sitting him down in front of one of the computers. 'Get everyone who's still in the city to come here. I've got a body full of Tithonian energy that's just itching to be used. Mega drained so much from these kids, it's only fair I give it back!'

'I…don't think they'll listen to anything I have to say,' Josh said, sounding ashamed, as he punched buttons on the keyboard and the words CityNet appeared on the screen.

'That wasn't you,' the Doctor said, putting a hand on Josh's shoulder. 'And they'll come. The Virus will take a few days yet to disperse. They'll be desperate, and desperate people will cling to any hope. Give them hope, Josh.' He flexed his neck and started jogging on the spot, as if it really did itch to have that lightning energy running through him.

Josh took on a determined look and switched on the microphone. 'Fellow citizens,' he started. 'This is your leader…Mega.' He stumbled a little on the name. 'For your safety and security, all citizens are requested to come to the hotel, where you will be safe from the Virus. Please help anyone who is already infected; everyone will be provided for, and together we can rebuild our City. Thank you for listening.' He switched off the microphone and gave a deep sigh, looking towards the Doctor and Donna for approval. Donna smiled encouragingly at him. 'What now?' he asked.

'Now we wait,' the Doctor said simply.

And so they waited. And the people came. Slowly at first, in dribs and drabs, and the party of three met them at the doors. The Doctor sought out those most infected with the Virus first, touching them on the forehead and releasing some of the life energy that the Tithonian had absorbed, bathing them in that soft amber glow that left them young again. The children looked on in amazement, and soon more came as the news spread. Donna could hardly believe that so many people had still been in the city, when it had seemed so deserted to her earlier. For the first time since she had arrived, the place seemed alive. Among the crowd she saw familiar faces, the two children from the mall, Dane and his tribe, with Isaac looking on in awe as his wrinkles were removed by the strange energy. Hours passed before the last child was cured, running away whooping for joy after the rest of his tribe. The Doctor gave his hand a shake to free the last tiny lightning sparks.

'Ah, that's better,' he sighed, flexing his fingers. 'And now, I think it's time we were leaving, Donna.'

'But you can't go!' Josh cried, and an echoing chorus joined him. Not all the children had gone back into the city. Dane's tribe and the two little girls had stayed behind, and now they pleaded with the Doctor and Donna to stay with them, to look after them. 'How are we supposed to survive on our own? The Mallrats were the ones who brought us all together, and now they're gone.'

'The Mallrats weren't superhuman,' the Doctor said. 'They weren't anybody special, just a bunch of kids like you, a Tribe. But they had a dream of a better future, and that's what made them different. You've got a tribe of your own now, Josh.' He gestured to the children around him. 'You promised to rebuild the City for them. What kind of city will it be? One where the tribes are fighting for power and chaos, like Mega wanted? Or one where you work together, to build something for the future, the way the Mallrats tried to?'

'I want a future,' Josh said emphatically. 'I don't want to fight anymore. Before, when I joined the Technos, I thought that trusting people only meant setting yourself up for betrayal, but I was wrong. I was wrong about a lot of things, but I can put it right now. For the city, for my brother.'

The Doctor nodded. 'Come on then, Donna. I think we can leave this city in safe hands.'

'Take care of yourselves!' Donna told the children, hugging them in turn. They were all surprisingly eager for it, even Isaac. It had been a long time since any of them had experienced anything resembling parental comfort. 'And each other!'

'Don't forget what the Mallrats worked for,' the Doctor called to them as he and Donna began to walk away. 'Keep the dream alive!' As they moved out of earshot he gave Donna a friendly nudge. 'I've always wanted to say that!' he grinned.

When they arrived back at the TARDIS, the Doctor harrumphed at the fresh graffiti that almost completely covered his ship. 'Well that's just charming,' he frowned. 'And when did they even have time to do it?'

Donna laughed. 'They're kids, Doctor!' she explained. 'Some things will never change! It's almost reassuring actually, after everything we've seen in this place.' They entered the ship, Donna hanging her coat on the railings of the walkway as they stepped down towards the control deck, the Doctor muttering about brakes. She untied the now balloon-less ribbon from around her wrist, which was now fleshed out again and as smooth as it had been prior to the Virus. She shuddered slightly at remembering it. 'Doctor?' she called.

'Mmhmm?' he answered absently, tinkering with a control panel.

'I know I'm not supposed to ask, but you said when we arrived that we hadn't gone that much further into the future. Are we going to be alright? Everyone in the present, I mean? My present? The Virus is a long way off yet, right?'

'Oh, you know me Donna,' the Doctor answered. 'Decades, centuries, millennia, it's all small change to me. Nothing to trouble yourself with!' He didn't meet her eye, but continued tinkering, and she wasn't sure if he was hiding something, or just occupied. She decided not to dwell on it.

'Course!' she laughed, and started playing with the ribbon in her hands. 'You know, I'll be needing another one of these, Doctor. I can't show up at the party without a balloon!'

The Doctor finished his repairs and smiled at her. 'Quite right!' he agreed. 'Did you know, there's a planet in the Oberon Spiral that uses balloons as currency? Last I heard they were in the middle of an economic boom, although I'm not sure if that's meant to be a good thing. Why don't we go find out?'

Donna cackled and rushed to join him at the controls, forgetting her earlier worries. 'Now this I have to see!' she whooped. 'Come on then, Spaceman! Allons-y!'

'Oh, you're getting good at this!' he winked. The two of them pulled their levers in unison and the TARDIS' central column worked like an outer-space bellows, creating that familiar whooshing noise that sent a tingle of excitement up Donna's spine. In the city streets, the police box began to wink in and out of existence, until finally it disappeared altogether, the noise gradually fading until all was silent once more.