'I don't know,' Anji said, 'but I think we should follow him.'

They heard the sound before they saw what was happening. Fitz overtook Anji at a run, to her evident surprise.

'Come on!' he yelled as they emerged into the daylight.

Across the courtyard, they were just in time to see the blue shape of the 'TARDIS the real real TARDIS fade and disappear from the sled. TARDIS fade and disappear from the sled.

'What the h.e.l.l?' Nesbitt said.

'He's run out on you,' Sabbath told them as he caught up, Lansing jabbing him with his rifle. 'Trying to save himself from the temporal effects.'

'Why don't you just shut it?' Anji said.

'Yes,' Fitz agreed. 'He's not running out on us. He's got a plan.'

As if in answer, the air was split by the tortured sound of the TARDIS engines as it solidified again in front of them. It was closer now. Within a few moments, the doors opened and the Doctor came out.

'Ah, there you all are.'

'Have you done it?' Anji asked. 'Whatever it is you were doing?'

'Well, sort of.' He seemed buoyant, bouncing on his feet. 'Work in progress, if you like.' He pulled something from his pocket and threw it to her. 'Here, catch.'

It was a lump of ice. Anji could feel it melting through her gloves. She held it up, and could see the tiny flicker of frozen fire deep within.

'I was too late to stop Curtis entering the time envelope,' the Doctor confessed. 'Which is probably just as well, since I've no idea how I might have done it.' He reached over and tapped the lump of ice. 'Tell them about o-regions, would you Captain Nesbitt?'

Nesbitt seemed surprised at the request. 'Er, well,' he bl.u.s.tered. 'They're parts of s.p.a.ce. Sort of.' He looked at Lansing for help.

'As I understood it,' the Corporal said, still keeping his rifle pointed at Sabbath, 'they are self*contained regions of s.p.a.ce so far removed from other areas that even the light from them has not yet travelled to another part of the universe. Isolated in the extreme.'

'Excellent,' the Doctor congratulated him. 'This light,' he tapped the ice again, 'as I told the military gentlemen here, is from one of those regions. But it travels slower than our own light. That is, after all, the point of all this, isn't it?' He waited for Sabbath to agree.

But the large man seemed disinterested. He was inspecting his fingernails. 'He'll be fifty years or more into the past by now,' he said quietly. 'Speeding up as he goes.'

'But this isn't just light, is it?' the Doctor went on as if Sabbath had not spoken. 'There's something living inside the light. Just as bacteria can survive inside ice. Not that this mattered to your plan. What did matter is that O-regions are constantly meeting and coalescing. As the light from one reaches another, it is no longer isolated.' He lifted the ice from Anji's hand. 'And because our light travels faster than the light in the region this came from, the O-region and the creature within the light inside the ice knew about us before we knew about them. In fact,' he went on, 'the light from Earth will have reached them centuries ago and with it knowledge of Curtis's nature.'

'Which means what exactly?' Fitz asked.

The Doctor was looking at Sabbath as he answered. 'It's the black hole that Curtis will become that has drawn this light here, isn't it? Just as any black hole draws light from wherever it can.'

Sabbath was smiling now. He nodded, as if to congratulate a slow pupil who has just caught on.

'Except, of course,' the Doctor continued, 'Curtis has not yet become a full black hole, has he? And for that to have happened in the past long enough ago for the knowledge to have reached the o-region he depends on the time machine in the ice cave. And that only works because of the properties of the light. The light which Curtis has not yet attracted because he hasn't yet become a black hole in the past.' He paused and glanced across at Nesbitt and Lansing. 'Do stop me if there's anything you're unsure about, won't you.'

'So it's a paradox?' Fitz said. 'In fact, it sounds like the granddaddy of all paradoxes.' He blinked. 'Sorry, forget I said that,' he murmured.

'More than that,' the Doctor said. 'It is a paradox that can only exist within a single universe.' He was talking to Sabbath again, and now Sabbath wasn't smiling. 'If you're right about how time travel works, which is after all what your whole scheme is predicated upon, then none of this can happen. Can it? If your theory is correct, then when Curtis travels back in time, that creates a new universe a new universe where a black hole was there at Time Zero. Otherwise we'd already be living in a universe Where that was the case. But the light that the black hole attracted is here in this universe not some other as yet uncreated one that you hope will supplant all others. It has to be for your plan to work.'

The way the Doctor was standing with his hands behind his back, Anji had an unhappy feeling he might have his fingers crossed. Lots them. 'So the universe isn't split by time travel?' she asked. 'Though I think my brain soon will be.'

'Demonstrably not.' The Doctor's hands appeared from behind his back, and he tapped Sabbath on the chest with his index finger. 'You're wrong. You've proved yourself wrong. Why don't you admit it?'

Sabbath's voice was like gravel. 'I admit nothing to you, Doctor.'

'Totally wrong,' the Doctor said gleefully. A trickle of water dripped down from his hand as the ice melted. 'About the nature of the universe, the nature of time, everything. Time will preserve the single timeline, the one real real universe, wherever and whenever it can. It doesn't spin off whole new ones at the drop or not of a cat. It forms loops within itself rather than a whole new universe. Then the time line coalesces round its chosen path, leaving ox*bow lakes of might*have*been, backwaters of what*if...' universe, wherever and whenever it can. It doesn't spin off whole new ones at the drop or not of a cat. It forms loops within itself rather than a whole new universe. Then the time line coalesces round its chosen path, leaving ox*bow lakes of might*have*been, backwaters of what*if...'

'I hope you know what he's talking about,' Trix said quietly to Anji. 'Is he always like this?'

The Doctor glared across at them both. 'Look,' he said, 'buying the Daily Telegraph Daily Telegraph rather than the rather than the Independent Independent has a very limited effect on the rest of the universe you know. Well,' he added with the hint of a frown, 'depending what you read in it, I suppose. But it forces the indeterminate state of " has a very limited effect on the rest of the universe you know. Well,' he added with the hint of a frown, 'depending what you read in it, I suppose. But it forces the indeterminate state of "Telegraph or or Independent Independent" rather than splitting off a whole new universe where that is the only difference. That's why Fitz's death or non*death didn't create a new universe our universe accommodated it within itself.'

'Are you saying that the whole of Quantum Theory or whatever is wrong?' Anji asked.

'Oh no. The universe splits when it has to, when there is no other way to resolve things.' The Doctor's face fell into shadow as he turned back to Sabbath. 'But it doesn't take the creation of a whole new universe, just to kill a cat.' The ice was all but melted in his hand now. 'And forcing together universes that have split for very good reasons can only have catastrophic consequences.'

From somewhere in the distance, it seemed, came the sound of gunfire and explosions. It was a strange, eerie noise. It seemed to be coming from all around them, yet it was still distant. Barely audible.

'You see,' the Doctor said. 'Or rather, hear. It's starting.'

'What is?' Trix asked.

'Echoes from the past. Look!' The Doctor pointed across, towards the main gates.

Nesbitt and Lansing both drew in breath sharply as they saw themselves, and the rest of the SAS group, running towards them. But like the sounds, they seemed insubstantial, shadows in the air. And overlaid, interspersed with them, Hartford's team was crashing into the courtyard. Both a.s.saults muddled and chaotic and overlapping.

'It starts with the events with greater pitch and circ.u.mstance,' the Doctor said, his voice barely audible above the ghostly firefights. 'But soon everything that has happened here, that will ever happen here, will be taking place at the same time. Not just the events from this universe, but from every universe and every possible universe. To be or not to be, in every sense.'

The Doctor watched the confused, shadowy images for a few moments. Then he stepped back and clapped his wet hands together. 'Luckily,' he said, 'there's an easy way to stop Curtis and the black hole and sort everything out.'

'There is?' Anji said.

'Oh yes.' The Doctor looked round at them. 'Right then, I need Fitz, Anji and George, please, with me. Captain Nesbitt, would you kindly look after Mr Sabbath until I get back?'

'I'll just wait here too, shall I?' Trix asked. The Doctor did not answer. 'Thought so,' she said. 'Just thought I'd check.'

Sabbath rubbed his chin thoughtfully as Anji and Fitz stepped forwards. 'So, you've got there at last, Doctor. I knew you would.'

'Worked it out you mean?' Fitz said. 'You can't keep him fooled for long you know.'

'No, I didn't mean that. I meant that, at last, the Doctor has come to the point where he has to face up to reality, in every sense. Can you do it, Doctor? The solution, as you say, is easy.' He took a step forwards, his eyes staring intent at the Doctor's expression. 'But can you do it?'

'I don't know,' the Doctor replied. His voice was quiet and devoid of emotion. 'I just don't know.'

'Why?' Anji asked. 'What's the problem?'

'The problem is that the Doctor now has to make a decision. It's all about choice, after all. No unresolved paradoxes, no indeterminates, no cheating this time.'

'What's he talking about?' Fitz said. 'What choice?'

The Doctor did not answer. He turned away.

'So much for all those speeches about free will and the sanct.i.ty of the individual,' Sabbath gloated. 'You wouldn't let me kill Nathaniel Ashe, and he wasn't even an innocent bystander. So what's it to be this time, now that there really is only the one choice? Will you save the universe, Doctor? Or sacrifice a single life?'

And walked silently, head down, through the ghostly fighting figures towards the solid reality of the TARDIS.

1: Greater Good

Monsters from an evolutionary might*have*been moved ghostlike through the castle. Hartford's incursion team attacked the Russian special forces. Again and again. Nesbitt's SAS stormed their way in with percussion grenades and lightning efficiency once more. George Williamson turned in a corridor, as if looking back at someone. But there was n.o.body there, which was ironic since he was not there himself.

Miriam Dewes fell dead to the floor in the Great Hall, Yuri Culmanov dying beside her. And Blake Michaels... At the same moment, Fitz pulled at the decaying tapestry and opened a secret pa.s.sage. He and the Doctor both entered it together, neither aware of the other, separated only by a tissue of reality; a trick of time.

An ice*TARDIS exploded yet once more.

And a little over a mile away, the real Fitz stepped out of the real TARDIS and stood once again in the ice cavern where he might or might not have died over a hundred years earlier.

It amazed Anji that George had not commented on the peculiar interior dimensions of the TARDIS. Perhaps he had seen and experienced so much recently, well, in the last century or so, that it just did not seem very remarkable. He stood, seemingly preoccupied, with Fitz and Anji at the console as the Doctor busied himself round the controls clicking his tongue and worrying switches.

Only now did it occur to Anji that while the Doctor had been out of her life for over a year, perhaps for him it had been only a day. Or, and given his apparent expertise now with the TARDIS controls, perhaps for him decades had pa.s.sed before he happened to find himself on a plane to Siberia with a man who was really a Cosmic Event.

The journey took only a few minutes. Then the Doctor opened the doors, and they stepped out into the ice cavern. At the far end, Anji could see a hole torn in the gla.s.sy wall. It reminded her of the window into the other reality, the other universe. She supposed it was where the SAS had blown their way in to get the ice*TARDIS.

'Right on target, for once,' Fitz said. He seemed to be in a sullen mood too, forever glancing sideways at George. Maybe he was disappointed that Miss fake*blonde MacMillan had not accompanied them, Anji thought. It had to be dyed. Didn't it? Not that it mattered at all given that the universe was about to end. But that colour didn't happen in the real world, whatever Quantum Physics might think. Without really thinking about it, she ran her fingers through her own hair, trying to tease out the worst of the knots.

'Yes, here we are.' Despite the urgency, the Doctor seemed hesitant, unsure quite what to say or do next, staring down at the frozen ground.

It was George who broke the silence. 'It's me, isn't it?' he said tightly. 'You have to kill me to stop Curtis. That's what Sabbath meant.'

'Oh George,' the Doctor said, quiet and sad. When he looked up his eyes were large and moist. 'I wish there was another way, I really do.'

'What?' Anji stared. 'You're going to '

But the Doctor cut her off sharply. 'I'm not going to do anything. It's entirely up to George. It's his choice. His life.'

'Some choice,' Fitz said grimly. He was looking around the cave, as if reminding himself how it looked. 'Wait a minute, Doctor,' he said. 'There's something wrong here...'

But the Doctor ignored him. 'If you die,' the Doctor said to George, 'If you really die rather than getting caught in the light beams, then the time envelope will cease to have existed. That's a possibility. In fact, I think that's why you're insubstantial now indeterminate. That may already have happened. In which case, Curtis will be able to get back as far as 1894. No further. At least, not by that means.'

'But he can still do it?' Anji asked.

'Yes,' the Doctor admitted. 'He can use the ice itself, or rather the light trapped within it. At the moment he's sort of piggy*backing on George. If George isn't around, Curtis will have to use the raw light source.'

'Great,' Anji said. 'Then what do we do? How does that help?'

'We destroy this ice cave,' the Doctor said grimly.

'I don't think we can destroy it, Doctor,' Fitz said. 'That's what I was trying to tell you. I blew this place to bits saw it explode. Yet here it is, completely intact. Just as it was.'

'He's right,' George said. 'A huge explosion.'

The Doctor was nodding enthusiastically. 'Exactly. The ice cave is destroyed, the slow light released by Fitz's grenade back in 1894.' He mimed the explosion by throwing his arms wide and making sound. 'The black hole that Curtis has become erupts from the broken ice that he was using, in which he was trapped like the light source. The energy is dissipated through time, right along the path he's been taking rather than consuming the world. And the blast is heard in Moscow, talked about and remembered until Tunguska eclipses it and the memories fade.'

'But how did you know?' Fitz asked. 'How could you possibly know back in 1894 when we met in St Petersburg, when you told me I'd find this cave and asked me to destroy it? Back then, how could you guess what would happen?'

The Doctor sighed. 'Oh Fitz, Fitz, Fitz. You of all people should know that.' He gave a weak smile. 'It might have been 1894 for you,' he said. 'But it was only about twenty minutes ago for me. When I found Curtis had already started his journey, I had no other option but to ask you destroy the ice.'

Fitz just stared. 'Ri*ight,' he said at last.

'But it hasn't happened,' Anji pointed out emphatically.

'Of course not. It can't happen until George forces Curtis to transfer to the ice and light, and the energy is there ready for Fitz to release, Until then it's in an indeterminate state. It has and has not happened, it might or might not have occurred.' He took a deep breath. 'The universe may or may not exist.' He turned to George. 'I really am sorry. But it's up to you. It's your decision, your choice that will determine what events really took place here. Whether the grenade just exploded, or the black hole was released with the ruptured ice and dissipated along the whole time path causing goodness only knows what side effects, but I hope preserving reality. And you must choose very soon. I can't do it for you.'

'I can.' Fitz's voice was low and as hard*edged as Anji had ever heard, His eyes were as dark as his tone. He stepped towards George. 'I thought he was my friend, but all the time he was deceiving me. He killed Galloway and Caversham.' Fitz was right in front of George now, staring into his faded eyes. 'A murderer's life for the whole of reality? There really isn't any choice, Doctor. Just tell me what to do.'

'No, Fitz,' the Doctor said quietly. 'George must decide.'

'I'm sorry, Fitz,' George said. 'About Galloway. But I didn't kill I Caversham.'

'You followed him and he just disappeared, is that it?' Fitz was shaking, and not just with the cold. 'Found out you'd killed Galloway, did he? Realised it wasn't me after all despite you letting me take the rap.'

'No I ' George shook his head, confused and upset.

'It wasn't George who killed Caversham,' the Doctor said gently. He put a hand on Fitz's shoulder.

'I went to look for him,' George said. 'I was walking along the corridor; I heard something and looked back. But there was nothing there.'

'Outside what is now the Cold Room at the Inst.i.tute,' the Doctor said. 'I've seen you do it. It was Curtis, Fitz. Hence the after*image of George in the corridor. And you found the evidence, remember. The pebble. I know, I read your journal.'

Fitz blinked. 'Read...? How could you?' He pulled the battered book from his coat pocket. 'It's here.'

'I know. Though you did lose a few pages.'

'They blew away,' Fitz remembered.

'You're in the British Museum, Fitz,' Anji told him, trying to lighten the tone, to snap him out of it.

'How could Curtis have killed Caversham?' he demanded.

'He tested the time envelope, found he could get back to before George was trapped in the ice.'