Yet, somehow, at the far end, was now a third door.

I marched towards it, but the man scurried in front of me with a funny little run.

'Please don't,' he begged. 'It may no longer be safe.'

'Safe? What do you mean?' I growled.

'It could be dangerous. It may not have been a moment ago. But now you've observed it, its behaviour may have changed. Perhaps one of the other two may be safe to open now. What about the one on the left?'

With skipping strides he rushed up to the door I'd originally chosen. s.c.r.e.w.i.n.g his eyes shut he twisted the handle, throwing it open with a flourish. His face split with delight. 'Aha! I was right look, another room!' He danced through, the door slamming closed behind him.

For a moment, I was alone again in that vast white room. Then the door opened just a crack. 'I forgot all about you, I'm afraid,' the man murmured dolorously. 'Come on in. The water's lovely. Only there is, sadly, no water.'

I stood in another large white room, the little man dancing at my side.

'Who are you?' I demanded.

'Me?' the man blinked with sad surprise. 'I'm the Doctor. At least I think I am. But that may not count here.'

'Where is here?' I was suspicious.

He threw his arms up. 'This? This is The Multivarium. And, antic.i.p.ating your next question, no one quite knows what one of those is. A lesson. Or a prison.'

'Are you my jailer?'

'Are you my jailer?' the Doctor snapped back, with cunning but no malice. He rubbed his hands together with the most ill-placed glee. 'Oh, don't answer. I can tell you don't know. Doesn't matter. We stand on a precipice of uncertainty.'

'Huh?'

'Didn't I just tell you? The Multivarium was created at an intersection between parallel universes. Literally anything is possible here... anything and nothing. If you see what I mean.'

'Frankly, no I don't.' I wasn't sure I liked or trusted this Doctor. He seemed to just expect people to put up with him.

He edged nervously towards a wall. 'I swear there was a door here earlier. Ah well, there'll be another one along in a minute. Or a year. Or at some point. Eventually. Probability, you see.'

He settled down cross-legged on the floor and breathed horridly into a musical instrument.

A long time pa.s.sed. Or no time at all.

A door was there. It didn't flicker into existence. It simply stopped not being there. The Doctor seized the handle excitedly.

'Is it safe?' I asked.

'Only one way to find out.' He stepped through. 'I've got a good feeling about this one.' He vanished with a wail.

There was a long, tense silence. And then a tiny cough.

I leaned over the lintel. Tumbling down into a vast white nothing was the Doctor's recorder. He was dangling over the abyss, clinging on to the door frame. 'It, ah, seems I was wrong.' He dangled there. I said nothing.

'I say,' the Doctor called. 'Can you help me up?'

The room had somehow stretched while we'd not been looking.

'Almost a corridor now,' remarked the Doctor. 'How many doors would you say there were? No, it's all right, don't bother to count. I think the number is close to infinity.'

'Don't be absurd. It's a hundred. Maybe two.'

'You think so, eh? I've always thought the human mind had trouble with anything above seven, but there we go. Two hundred doors. Maybe.' He rubbed his hands together happily. 'Which one shall we choose?'

'We don't have to choose any of them.'

'Aha! You've hit upon the heart of the Multivarium.' The Doctor settled himself down on the floor, stretching his legs out. 'All right then. Let's choose nothing.'

He lay down, and contemplated the ceiling.

'Could you stop that?' I asked exasperated.

'Stop what?'

'The humming.'

'Oh, I am sorry!' The Doctor sat up. 'Just slipped into it. How much time do you think has pa.s.sed?'

'I don't know,' I snapped.

'Neither do I.' The Doctor frowned. He sucked a finger and held it up to the air. 'No air flow. What a pity. Would have been rather helpful.' He stood and dusted himself theatrically down. There was no dust here. 'Maybe we should choose a door. Might break things up a little...' He threw himself at a door handle before I could stop him. He pushed it down, letting it linger there a moment. 'After all, what's the harm?'

He opened the door. Huge flames poured out.

The Doctor slammed the door shut.

'I knew that was going to happen,' he p.r.o.nounced, sucking his burnt fingers.

'Why should we open any of the doors?' I pressed him again on this.

We were walking the length of the room. There were a lot more than two hundred doors.

'Don't you want to get out?' The Doctor sounded petulant. I didn't reply. I wasn't sure any more. 'One of these doors, just one of them, holds the possibility of escape. But even then that's not a fixed probability. Merely looking at one door for a moment too long might allow the exit to sidle away elsewhere. Escapes are crafty things,' he chuckled.

He was infuriating. Like a child who'd never grown up. A wise old man pretending to be a clown. I wanted the Doctor to go away. I also knew he was the only company I'd find here.

'The world in a door,' he mused, leaning his face against one. He beckoned me over. 'Can you hear anything?' he whispered. 'Something scratching?'

'No.'

'What's behind this door?' He stepped back, forlorn. '"Shall I walk along the beach, Do I dare to eat a peach?"' He turned on his heel and went away with his little waddling walk.

I'd had enough of him. I opened a door and stepped through.

The room was small and peaceful. It felt very calm in here, the air unnaturally still.

A worrying thought fluttered up. Why was I here?

It didn't matter. Not now I'd found this room.

I closed my eyes.

I woke to find the Doctor leaning over me, patting my cheeks.

'There you are! Oh, thank heavens.'

I was in a different room. Bigger. Six doors.

'You are very lucky I saw where you'd gone and followed. Had to think on my feet. Air's a potent mixture of oxygen and nitrous oxide. Most soporific. I hadn't considered that. But of course, an infinite variety of atmospheres is possible. And, of course, radiation... imagine that. A room that looks and feels normal but contains a minute particle of uranium. We could have been exposed to that already and now it's too late. Oh dear.'

He stopped talking and looked gloomily around. He mopped his brow with a spotted handkerchief.

'I wonder what happens...' The words were pouring out of him now. '... when two atmospheres meet? Under the right conditions you could open a door, the elements would mix and BOOM!'

Before I could stop him, he flung open another door.

There was no explosion.

Dust was drifting down through it.

'I wonder how long that's been happening.' He didn't notice my fury.

We edged through, our feet leaving prints in the dust. 'Try not to breathe too much of it. Just in case.'

We reached a door at the far end and he looked back. 'Footprints!' he exclaimed. 'Kilroy was here.'

We went into the next chamber. When I looked back, our footprints had gone.

'How did we get here?' I asked.

The Doctor contemplated the ceiling.

'Well, that's the thing about the Multivarium. Just as it's almost impossible to leave it, it's utterly possible to enter it. I was on the Moon. Zoe and Jamie and I were dealing with the Ice Warriors. I opened a door...' He wrinkled his nose. 'Or did I? I'd like to get back. Dread to think how they're getting on without me. Perhaps this is an Ice Warrior trap... Seems a bit subtle for them. What about you?'

'I really don't know,' I said. 'I have a memory of falling.'

'Not very helpful.' The Doctor was accusing. 'Perhaps you've been here no time at all? Or a very long time. My jailer, watching my every move. Subtly influencing my behaviour.'

I couldn't remember. I panicked at that, answering him hotly. 'I can a.s.sure you I'm not. It's far more likely to be you. You're the one who keeps putting us in danger.'

'Well, there is that,' he conceded. 'Perhaps we are each other's keeper. That's a thought. Unless you suddenly remember you're an ancient ent.i.ty that likes to imprison people,' he wrinkled his brow. 'You're not are you?'

'No,' I said, firmly.

We walked on.

We walked on some more.

We did not get hungry.

We did not sleep.

We just kept walking.

We stopped walking.

We sat down.

We waited.

We waited until we got bored of waiting.

'A chair,' sighed the Doctor.

Neither of us had spoken for some years.

'I do rather fancy a chair,' he repeated. 'Never really cared for them before. They just happened. But in all the rooms we've seen, no chairs. Somewhere in this infinity there must be a chair. A room that is just chairs. A room of chairs that are too small to sit on. A room where you need a Sherpa just to climb onto the seat. All chairs are out there somewhere ' he chewed his lip 'and yet we've not seen any of them.'

'Now that you've mentioned it,' I smiled, 'perhaps the next door we open will have a chair.'

'Maybe you're right,' the Doctor agreed.

We had a week of opening doors. We'd suddenly taken to it again.

Many of the rooms had been identical, but the Doctor had a.s.sured me that, like snowflakes, no two rooms were quite alike. One room had been so short we'd had to crawl through it. Another had clouds. We liked that one.

Our present room was unremarkable.

We had opened the doors. The rooms beyond were the same again.

As we had closed each door, the Doctor had marked it with a stick of chalk. He had developed this habit recently. It did not matter as, once through a door, we never encountered the markings again. You could open a door one moment and it would show a huge s.p.a.ce with six doors. You could open it again and there would be nothing but a cupboard soaked with fresh rain.

'Why do you bother?'

'Force of habit,' he said, and then stopped, repeating the phrase as though it was important. He stopped making his marks shortly after that.

We stepped through a doorway.

Suddenly I was pulled back, the Doctor grabbing hold of me.

We were crammed in the doorway.

'Mrph?' I said. My voice seemed odd.