'You mean, this area has grown old and decayed?' Kornick said. He stared into the gloom, his expression a mixture of scientific curiosity and horror.

'We should head back,' Archan said. 'There's nothing out there, except maybe more of the creatures that killed those people.'

'Certainly no mercury,' the Doctor murmured.

They retraced their steps to the reception area. The wolf-like creature was still unconscious, drawing rasping ragged breaths. They picked their way warily around it.

'We should try B Block,' Kornick decided. 'That's where Malatan Benervan and her team are working. If they don't already know about these animals, we need to warn them.'

He led the way through the wreckage to another door. The electronics had failed completely, so they had to prise the door open with their hands. The lights in the corridor beyond were on. It would have seemed perfectly normal were it not for the viscous grey-green liquid that stained one wall. It dripped from the ceiling, pooling in the middle of the corridor.

'It's leaking through,' the Doctor realised. 'What's above here? Some sort of storage tank?'

'Just another lab,' Lizbet told him. 'Where Malatan and her team are working.'

'Maybe something they were working on has spilled,' Archan suggested.

'We should check they're all right.' Kornick didn't sound optimistic.

'What's that?' Dalla said, pointing down the corridor ahead of them.

It was like smoke, drifting slowly towards them. Thin and pale, almost ethereal, coiling and extending.

'Escaping gas of some sort?' Archan wondered.

'Don't breathe it in,' the Doctor warned. 'Just in case.'

They held their breath, and hurried through. From somewhere nearby a voice called out, the words indistinct as if lost on the breeze.

'What was that?' Lizbet said, turning.

'Must have come from the lab above us,' Kornick said. 'We should get up there.'

The lifts weren't working, and they had to force open the door to the emergency stairs. The Doctor held the doors open while the others squeezed through the gap. He eased himself after them, and the doors clanged shut again behind him. Kornick was looking pale, leaning forward with his hands on his knees and breathing heavily.

'Are you all right?' the Doctor asked.

Kornick straightened up, his face drawn. 'Of course I'm all right. We need to find Malatan.' His eyes were ringed with red, as if he'd been rubbing them. Maybe they'd been irritated by the smoke, or whatever it was.

The lighting fizzed and flickered as they made their way up the stairs. Sections of the handrail were rusted almost through, yet next to them were sections of untarnished steel. The stairs too were a juxtaposition of polished elegance and fractured, crumbling stone.

The doors were gone, along with a large part of the lab. No rubble or debris, just nothing. Like those sections had never been built. Wind scattered papers and raked through broken gla.s.s as the Doctor and the others picked their way through the mess. Double doors at the other end of the room stood in the middle of an incongruous section of wall.

There were pools of the grey-green sludge on the floor. One metal stool was covered with it. The Doctor dipped the end of a rusty stylus into the slime, sniffed at it, and grimaced as he realised what it must be. He dropped the stylus into the viscous liquid and straightened up.

As he did, he spotted a store cupboard, standing alone against a wall that was no longer there. The doors opened easily, but the contents were a mess of smashed jars and broken test tubes. Sand that used to be gla.s.s was scattered across one shelf. A viscous silver stream ran fluid along another. Mercury. He didn't need a lot and there was just about enough. The Doctor pulled an old, battered toffee tin from his pocket and coaxed the mercury to flow and drip into it.

'What happened here?' Dalla asked, her voice barely more than a whisper and almost lost in the sound of the wind.

'Time distortion on a ma.s.sive scale,' the Doctor began as he pressed the lid tightly back on the tin full of mercury. But he got no further.

The doors opposite them burst open. The wail of the wind became the roar of the creatures charging towards them. This time, no one hesitated. For once the Doctor didn't need to shout 'Run!' because they were already running.

They clattered down the stairs, growls and roars echoing down after them. Back to the corridor below, desperately wrenching open the doors again, and squeezing through the narrow gap.

'Give me a hand,' the Doctor gasped.

Lizbet and Kornick were already off down the corridor. But the two students stopped to help the Doctor force the doors shut again. A hairy arm rammed through the gap, claws scratching for them as the creature tried to force its way through. Dalla took her shoe off and hammered at the arm with the heel until the creature drew back. Then the Doctor and Archan slid the door fully closed. The Doctor's sonic screwdriver fused the lock. But there were already dents appearing in the metal as they hurried after Kornick and Lizbet.

Lizbet was standing just inside the reception area, hand to her mouth, staring ahead wide-eyed.

'What is it?' the Doctor demanded. 'Has the creature woken?'

She shook her head, unable to speak, just pointing. Her hand seemed insubstantial, strangely translucent. A trick of the flickering light, perhaps.

But what she pointed at was no trick. Kornick was in the middle of the room, tearing at his lab coat with clawed hands. His face was a ma.s.s of matted hair, eyes receding and features blurring.

'Help...' he pleaded, 'Help me.' But the words became a guttural roar of sound.

'What's happening to him?' Dalla gasped.

'Regression,' the Doctor replied quickly. 'Let's get past him before the process is complete,'

'But we have to help him,' Archan said.

'Too late,' the Doctor told him. 'He's been caught in a residual time distortion. It'll happen to us all if we hang around. I thought the effects had dissipated, but I was wrong.' He sighed. 'Sorry.' Then he led them at a run across the room, keeping clear of Kornick. The scientist watched them through rheumy eyes. A trail of saliva trickled down his inhuman chin.

'We're going back to the Nihilism Chamber?' Dalla asked as the Doctor led them down the corridor.

'The only place that's safe.'

From behind them came a roar of anguish and rage.

'And probably not for long,' the Doctor added.

They quickened their pace. The door behind them crashed open, and they broke into a run. None of them turned. They could all hear the growling anger, the claws scratching on the floor as the creature that had been Kornick bounded towards them 'Hang on,' Archan said, breathless. He risked a look over his shoulder. 'Where's Lizbet?'

The Doctor looked back too. A thin mist, like smoke drifted down the corridor after them, blurring their view of the slavering creature as it approached. 'So, not a trick of the light after all,' he murmured.

The smoky cloud caught up with them, keeping pace. The voice was faint, as if it came from another room. 'What's happening to me?'

'I'm sorry,' the Doctor said. He pa.s.sed his hand slowly through the misty air as he ran. 'I am so very sorry.'

'You can't help me?' The voice was fainter.

'It's too late.'

'Then help the students. Get them to safety.' The last words were all but drowned out by the roar of the approaching creature.

The smoke thickened, coalescing as if gathering itself in a misty curtain across the corridor. Through it, the Doctor could see the dark shape of the Kornick-creature bounding towards them. He stood transfixed, the two students either side of him.

'We'll never outrun that thing,' Dalla said, her voice trembling.

'I'm hoping we won't have to.'

'What do you mean?' Archan asked.

The Doctor didn't need to answer. The creature hurled itself at the wall of hazy smoke, claws ripping down through and scattering it. Suddenly the air was alive with light and sparks. Illuminated in the middle of it all was Lizbet an image of light picked out in points of fire. Then the air cleared, and she vanished with the last residue of smoke.

The creature too was on fire glowing with an inner light that ripped out through it. With a howl of anger and pain, it shimmered and faded. For a moment, Professor Kornick was there again, staring back at them, then he was gone.

'Forget the Nihilism Chamber,' the Doctor said. 'Back to the TARDIS.'

He grabbed Dalla's hand, and she grabbed Archan's. Together they hurried down the corridor.

'What happened?' Dalla demanded. 'That smoke?'

'It was Lizbet, wasn't it?' Archan said. 'She's changed. Like Professor Kornick. But then she came back they both did.'

'And now they're gone,' the Doctor said. 'I'm sorry. One regressed, the other advanced.' His words were breathless and urgent as they ran. 'Like Malatan and the scientists in the lab upstairs, regressed through time to primordial sludge. Like this whole facility parts of it aged to ruin, others not yet built.'

'But, Kornick and those creatures?' Dalla gasped.

'An evolutionary blind alley. What humanity never became.'

He bundled them through the door to the Chamber.

'And Lizbet?'

'What the human race may one day evolve into. Ethereal beings, all mind and no substance. When they met, they cancelled each other out. Two might-have-beens that ultimately never existed.'

'Will it happen to us, too?' Archan asked, ashen-faced as he took deep breaths.

The Doctor unlocked the TARDIS and shoved the two students inside. 'Not if I can help it,' he told them.

They stood inside the doors, looking round in amazement. But the Doctor's attention was drawn to the roof, the view depicting the sky above them. The sun, darkened and blotched slowly shimmering as if with suppressed energy.

'Just in time.' He ran to the console, closed the doors and fumbled in his pocket for the tin full of precious mercury.

Moments later, a door burst open and creatures that evolution never created stumbled into a corridor. Intelligent smoke drifted away towards the reception area. A wheezing, rasping sound split the air as the TARDIS dematerialised. And high above all that, the sun of Rontan 9 exploded into a supernova, millions of years ahead of schedule.

'What happens now?' Dalla asked as they watched the spectacle unfold on the TARDIS scanners. Crimson, orange and yellow stained across the darkness of s.p.a.ce.

'"Now" no longer has any meaning here,' the Doctor told her. 'But now I'm going to take you home. Or at least, somewhere stable and safe.' He glanced up at the roof, burning with a stellar explosion. His expression was grim. 'And then I have work to do.'

'There was a war a Time War. The last great Time War. My people fought a race called the Daleks for the sake of all creation. And they lost.'

The Tenth Doctor, Gridlock (2007)

On television, Doctor Who has only given us glimpses of the Time War fought between the Time Lords and the Daleks. In different stories, we've learnt that the home worlds of the Autons, Gelth and Zygons were among those planets lost in the conflict. In The End of Time (20092010), we see smashed up Dalek s.p.a.cecraft littering the Time Lord home planet, Gallifrey.

We finally witness some of the actual fighting in The Day of the Doctor (2013), with Daleks moving through the Time Lord city of Arcadia. But the focus of that episode is well away from the conflict, based in the shack where the War Doctor faces a terrible dilemma. Because of the war, he says, 'Every moment in time and s.p.a.ce is burning.' He alone can end the conflict by using a special Time Lord super-weapon called the Moment but if he does so, he'll destroy all the Daleks and Time Lords, too, including the 2.47 billion Time Lord children on Gallifrey.

Now, that's a lot of Time Lords and Daleks to destroy in an instant, an act of appalling double genocide. But when the choice is between them and all the rest of the life in the universe, what the Doctor must do is clear isn't it?

The decision he makes at the end of the Time War haunts each of his subsequent incarnations, perhaps most notably in The Parting of the Ways (2005) when the Ninth Doctor is offered a similar choice again. But the choice the War Doctor is offered, his response to it and the decision he finally makes are all indicative of a broader question: the relationship between science and war.

'That's typical of the military mind, isn't it? Present them with a new problem, and they start shooting at it.'

The Third Doctor, Doctor Who and the Silurians (1970)