'Gravity differential,' he said. 'That room has too much of it. You could see the door slow as it opened. We'd have been crushed. We have to stay here.' He was gripping the frame tightly. 'Go forward and we'll die. Let go and we'll be swept up.'

'By what?' I was finding it hard to hold on to the Doctor.

'The pressure between the two rooms is evening out. Like a tide. For a room to have its own gravity is nonsense, of course. But...' He gingerly let go with one hand and pushed it into the room. 'Ah yes... heavy, but not too bad.'

He let go of me, and we edged into the room. It was like wading through a bog.

'We should hurry,' he said as we slumped forward. 'It's tidal, which means the force will return and squash us like pancakes.'

We noticed the same patterns of rooms repeating, becoming more frequent. Closing in.

For a while that became a game. Could we predict correctly whether the room beyond would be some variation of fire, rain or gravity?

That amused us for a time. Until the mere act of opening a door became a ch.o.r.e.

'Are you all right?' I asked the Doctor.

He was lying on the floor again. These days he no longer bothered even leaning against the wall.

'Perhaps,' he murmured, 'you are the Doctor.'

'Me?' I was annoyed. Did that mean he expected me to do something? I just wanted to lie here for ever.

'Maybe I'm keeping you here. You should be out there...' The little man raised his hands. Once that gesture would have taken in the universe. Now it just stroked the wall gently. 'Doing things. Struggling against impossible odds. Instead of which... I'm keeping you trapped in this small room.'

'It's not that small,' I began.

'They're all small!' The Doctor scrambled unsteadily to his feet and stood there, shaking. His clothes were little more than rags, his hair long and matted. He flung open a door and marched through. I could hear his voice echoing from the room beyond. Wearily I traipsed after him.

'It's not so small,' I said of the echoing s.p.a.ce.

He did not reply. He simply padded through the room, opening doors and letting them fall shut.

He reached a wall and sank against it.

'There may as well not be any doors.'

'Long ago...' began the Doctor.

He had not spoken for a long time and his voice was a croak. He cleared his throat.

'Long ago, I said this may be a prison. Or a lesson.' The Doctor was lying on the floor, his foot kicking out lazily against a door. Tap. Tap. Tap.

'I think I've finally learned the lesson.'

Tap. Tap. Tap.

The Doctor said nothing.

Tap. Tap. Tap.

If he was waiting for me to speak, I didn't care. I didn't care about anything any more.

Tap. Tap. Tap.

The Doctor kept on with his futile kicking against the door. One door. One door in a room with all the doors. A door that would not give way. No matter how many times he kicked it.

'Stop that,' I snapped eventually, after a day or two had pa.s.sed.

'Shan't,' the Doctor was truculent. He carried on kicking the door.

'What are you hoping to achieve?' I asked.

The Doctor just laughed, a sad and weary chuckle.

His shoe, scuffed now, carried on tapping.

'We are in a room. A room full of doors. Beyond each door is another room. Universes and universes of possibilities. It's a perfect, infinite trap. And yet... The doors are gateways. I am hoping to wear one down. To make a mark. Then the system will be, just a little less perfect. That's all.'

'That's all?'

The Doctor finally, wearily stopped tapping the door and gazed up at the empty ceiling.

'The lesson of the Multivarium is this: it shows you how pointless any action is. Whatever we do, whatever door we open, it is just one choice in an infinity of possibilities. Somewhere else, someone else does the opposite.' The Doctor stood up and looked around himself as though seeing the room for the first time. 'I am here to be humbled. To learn my lesson. To do nothing. To not interfere. But...' He indicated the door. A tiny amount of shoe-leather remained against the edge of it. 'Some of us just can't help leaving our mark.'

This should have been the saddest moment in our lives. Caught in an eternal prison, unable to do more than scuff a door.

Instead, I felt a surge of hope. The Doctor seemed to gather strength. I've no idea where he got it from, but he chuckled. He winked at the doors as though to say, 'I know what you're up to, my friends, and it won't work.'

The Doctor rubbed his hands together.

'Come on,' he said, wrapping an arm around my shoulder. 'We've got work to do. My friend Zoe would love this. We need a system a methodology. She'd approve. She loves science.'

We examined the doors. The handles were uniform and grew out of the doors themselves. There were no screws. Similarly the hinges joined to the frame with impossible smoothness.

'Too easy.' The Doctor shook his head, ruefully awarding someone else the points.

We carried on walking. We negotiated the familiar rooms like old friends. The cold room. The oxygen room. The room with no floor. The room with heavy gravity. The room of clouds. The room that smelt of flowers. The rooms that had many doors. The rooms that had only one. The room where thinking changed the colour of the walls. The rooms where speaking hurt.

As we went, the Doctor started marking the doors again.

'Oh, I know,' he chided me. 'But previously, ah, that was meddling. Now I have a system!'

He carried on. A little mark for fire. Another mark for rain and so on.

Soon the symbols began to reproduce themselves on the doors. Not all the time. But sometimes.

'We're changing things,' I remarked, astonished.

'I rather think it's meant to mock us. But we are simply breaking the rules through playing by them.' The Doctor winked as though he'd made a little joke.

The room had four doors. We reached it by walking through a room where the air was argon. The Doctor looked around and smiled.

'This will do splendidly,' he grinned. 'A final lesson for you.'

'A final lesson?'

'Indeed.' A birdlike nod. 'The theory of multiple universes is smug. I don't care for smug things.' He looked more than a little smug himself. 'Suppose you say, "I don't believe in multiple universes." Well, "Aha!" someone snaps back. "That's all very well. But even if, in this universe, multiple universes don't exist, then in another one they do!" And then comes the nodding. I do hate the nodding.' He nodded.

He turned to each of the doors. 'The Multivarium puts you firmly in your place. Teaches you not to interfere because it is an exercise in futility. But if you never try then no one ever wins. There is no way out of the Multivarium. Except that, if there is no way out of the Multivarium, then, obviously, in another room there must be a way out.'

It was nonsense. All that excitement and now he was just babbling. He'd gone mad. I was stuck here for ever with a madman.

Then I noticed the doors around us. All of them were labelled with the familiar signs. Except for one. I stared at it. Amazed.

'What now?' I asked the Doctor. 'Are you going to clap your hands and go home?'

'I could,' he conceded. 'But where's the fun in that? I do rather prefer a decisive victory.'

I tried not to stare again at the new door. Scuffed and a slight shade of blue. He indicated that I should try the handle. I did.

'Locked.' My voice was a whisper.

'Splendid.' He beamed. 'I hope you don't mind a show-off.'

'Not at all.'

The Doctor indicated the door we'd come through. Argon, an inert gas. Which he'd closed hastily.

He'd left ajar a door marked with his sign for 'heavy gravity'. It was steadily, wrongly, creeping into the room. I lifted my foot. It left the floor grudgingly.

'Would you care to open the door marked "Oxygen"?' He offered it to me like an ice cream at the seaside. The seaside! I could remember that. And ice cream. Food. How long since I'd eaten?

My heart beating faster, I opened the door.

'We'll have a little longer to wait because the heavier gravity impedes the Brownian motion of the molecules, but it'll be worthwhile in the end, you'll see.' He no longer cared that I didn't understand. He was just so very pleased with himself. 'We're waiting around for our probabilities to meet. We're creating an oxygen-rich environment.'

We waited.

Then the Doctor indicated I should take a deep breath and stand in the room full of argon.

'A little aside from the door. Just in case I've made a mistake.'

I did so.

I saw him scamper over to the room marked with the sign for fire and fling it open.

Then I saw him running towards me.

Then there was an enormous explosion.

The Doctor was lying on the floor of the inert room. He stood up, clearly desperate to breathe. I'd not had time to close the door as he'd flown through. The ball of fire had ballooned in, almost reaching him, but had then washed out again like a tide.

Equilibrium ruled in the room with all the doors.

We stepped back out into the chamber. It was a burnt-out wreck, the doors that led to gravity, oxygen and fire swinging limply from their hinges. They didn't matter any more.

We just stared at the blue door. It had gone. Blown away. Beyond it lay darkness.

'I think I've made my point,' the Doctor said slowly. 'There's always a way.'

He led me to the doorway. To the darkness which seemed suddenly so uncertain.

'I've no idea who you are. I've no idea who I am really,' he said, his hands in his pockets. 'Maybe you're the Doctor, maybe I am.'

'I think you are,' I smiled. 'What's out there? Is it the way out?'

We stared through the exit into the blackness beyond.

'I have absolutely no idea,' the Doctor grinned. 'Shall we go and see?'

'Sideways in time, across the boundaries that divide one universe from another.'

The Seventh Doctor, Battlefield (1989)

The TARDIS can travel anywhere in s.p.a.ce and time and even to other universes, of several distinct kinds...

When Doctor Who began in 1963, the intention was to have three kinds of stories. The first two are obvious: historical stories in which the TARDIS travels back in time through Earth's history, and science fiction stories in which the TARDIS travels forwards into the future or away to alien worlds. The first season more or less alternates between these two types: historical (the caveman bits of An Unearthly Child, then Marco Polo, The Aztecs, The Reign of Terror) and science fiction (The Daleks, The Keys of Marinus, The Sensorites).