'You're not getting out of there till O'Keagh comes back.'

'I understand that. You're missing my point.'

'Which is?'

'Well, why?'

'Why are you in a box?'

'No, I know know why I'm in a box. I have been why I'm in a box. I have been put put in a box. What I want to know is why it bothers me.' in a box. What I want to know is why it bothers me.'

'It would bother anyone, I imagine.'

'But it bothers me particularly.'

'How do you mean?'

'Well, panic and such.'

'Ah, I see. You're phobic.'

'Exactly.'

'And you don't know why.'

'I don't, no.'

'Probably something that happened in your childhood.'

'I don't remember my childhood.'

'Many people don't.'

'And how do they explain it?'

'One doesn't really need a specific memory. The symptoms themselves symbolise what the experience probably was. More importantly, they reflect the underlying emotional reality the experience has become.'

'Such as?'

'The conviction that you're shut in with something you can't get away from.'

'And what would that be?'

'Well,' there was a shrug in Chiltern's voice, 'what else is in there with you?'

Terrific, thought the Doctor morosely. Insight from a man who had him imprisoned in a box. As if the situation weren't demeaning enough. He bet Sabbath never got shut up in boxes. Too big for one thing he broke off this piqued line of speculation at the sound of O'Keagh's returning footsteps.

'All taken care of, sir.'

The Doctor shut his eyes in a pang of reluctant sympathy for Scale. And guilt. Vera was right. In a way, he'd led him to his death.

The lid banged open. Before the Doctor could move, O'Keagh grabbed him, wrenched his arms up behind his back, and hauled him out. For the first time, he took in his surroundings. Steel drawers and cabinets. Shelves of large, labelled jars. Electric lighting above a long metal table with straps hanging from it. A white-tiled floor with a large central drain. The Doctor's eyes winced away from this back to the jars. They definitely contained tissue, but he couldn't make out exactly what kind. O'Keagh dragged him towards the table.

The Doctor supposed things could be going worse, but for the moment exactly how eluded him. 'What do you think you're doing?' he rasped at Chiltern rather stupidly, he reflected, as the answer was unpleasantly obvious. 'Are you going to kill me?'

'I don't know yet,' said Chiltern as O'Keagh wrestled the Doctor on to the table. Chiltern stepped forward to fasten the restraints. Once his wrists were strapped, the Doctor stopped fighting. He closed his eyes in exasperation. Chiltern moved down to secure his ankles. 'Thank you, Mr O'Keagh. That will be all for the moment.'

The Doctor heard O'Keagh leave the room. Chiltern came back to the head of the table and regarded his prisoner thoughtfully. 'I've been told you have dangerous eyes. Will you behave, or do I have to blindfold you?'

The Doctor sighed irritably. 'You don't need to worry.'

'Good. Now, who are you?'

'I'm the only one who can help you.'

'Do I need help?'

'Badly.'

'Really? I'd say you were the one in trouble at the moment.'

'However much trouble I'm in, there's only one of me. How many of you are there?'

Chiltern stared at him rigidly. 'What are you talking about?'

'Oh, don't be coy!' the Doctor shouted, jerking at the straps. 'I'm talking about the machine! The time time machine! Now are you going to release me and let me help you, or not?' Chiltern took a step back. 'You have the mirrors, but the mirrors aren't enough. You have to have the machinery. You must have found it separated from the mirrors. How did you even know what it was? Surely there wasn't an operating manual. The thing must have been all set up, with the frame for the mirrors intact but empty. What was it being sold as?' machine! Now are you going to release me and let me help you, or not?' Chiltern took a step back. 'You have the mirrors, but the mirrors aren't enough. You have to have the machinery. You must have found it separated from the mirrors. How did you even know what it was? Surely there wasn't an operating manual. The thing must have been all set up, with the frame for the mirrors intact but empty. What was it being sold as?'

'A carnival ride,' Chiltern whispered. 'But no one could guess how it had worked.'

'I'll bet.'

'So they'd sold off the mirrors for a maze. I knew as soon as I saw it that it was unique, some strange work of genius. Then the man who had it, a dealer in antique machines, told me of the queer things the mirrors had sometimes showed. I traced them.'

'You'd guessed it was a time machine.'

Chiltern nodded slowly. 'I knew it was something extraordinary, and thought it might have to do with time.' He came closer. 'I guessed. But you knew.' He bent over the Doctor. 'How did you know?'

The Doctor didn't answer. Chiltern placed a hand on his forehead, then felt for his pulse. 'There's something unnatural about you. Your body temperature is much too low, for one thing.'

The Doctor had been thinking for decades that he ought to devise an explanation for that particular peculiarity. Unfortunately, he still hadn't come up with one. Chiltern took a stethoscope from his pocket and undid a few buttons of the Doctor's waistcoat and shirt. He frowned. 'What happened here?'

'Accident,' said the Doctor shortly.

'A rather complicated one, apparently. What about this?' Chiltern fingered the thick scar above where the Doctor's heart had been. 'You haven't led a very healthy life.'

'That's one way of putting it. So you're a surgeon too?'

'I do the occasional autopsy.'

'I see. And send the occasional corpse to potter's field. So much for Scale.'

Chiltern shrugged and pressed the cold stethoscope to the Doctor's chest. He listened for a few seconds, moved it, listened again. 'That heartbeat certainly doesn't sound normal.'

'No,' agreed the Doctor drily. He turned his head to look at the jars. Each contained a grey, ridged brain. Chiltern took his chin and turned his head back, brought a lit match towards then away from his left eye and then his right.

'Abnormal pupil response.'

'Excuse my asking, but what exactly are you doing?' Chiltern walked out of sight. 'Did Scale bring me here because you needed some kind of new specimen?'

Chiltern returned with a hypodermic. 'I'm just going to draw some blood now. Your coat's in the way, so I'm going to take it from your neck. Please lie very still.' The Doctor lay very still. Chiltern slipped the needle in with painless, professional ease. 'Your blood's a bit orange, isn't it? Rather subtle, but definite.' He held the hypodermic up to the light. The Doctor looked at his blood.

'Which one are you?' he said. 'Nathaniel or Sebastian, the Good Twin or the Evil Twin? Not that I really need to ask.'

Chiltern smiled thinly. 'You've guessed.'

'It wasn't that hard. Your twin has a completely different personality. Or should I say, your enantiomorph? Your mirror twin.'

Chiltern's eyes went so pale they were almost colourless. 'You know a lot, don't you?'

'More than you can possibly imagine. You were the first to try the machine, weren't you? After that, you changed the settings. You tried it on at least two more people, and it was still a disaster. But it's a different sort of disaster with you. You and Nathaniel experience the world differently, independently, as if you really were two distinct persons. And he doesn't seem to have a complete set of memories, or an accurate set either.' Chiltern didn't answer. 'He didn't know the truth, did he? Did discovering it drive him mad?'

Chiltern stood motionless for a moment, no expression on his face. Then he moved out of sight again. The Doctor was fairly certain by now that the only reason he was tied to an autopsy table was that it was the easiest place to secure him, but he still didn't find it reassuring. Probably it was the view of the brains that was getting him down. 'You do realise I'm probably the only person on Earth who can help you?'

Chiltern returned. In his hand was a microscope slide smeared with the Doctor's blood. 'Only because you're not from Earth.'

'Oops,' said the Doctor. 'Rumbled.'

'What exactly are you?'

'It depends. Sometimes I'm a wave, sometimes I'm a particle.'

Chiltern stared at him. 'You're very strange. Even apart from not being human.'

'Yes, I've heard that. Now that you know my mysterious secret, how about unstrapping me?'

'Not just yet.'

'Oh for heaven's sake,' said the Doctor irritably, 'I can help you.'

'Yes.' Chiltern put the slide in his pocket. 'Scale told me you asked him about the mirrors. How did you know to do that?'

'I was at the carnival. He babbles about them to anyone who will listen.'

'How indiscreet. Well, he won't any more. Was it beings such as you who made the machine?'

That hadn't occurred to the Doctor. He supposed it could be true, but since he actually had no idea, he said, 'No...'

'That means there are other civilisations out there capable of having made it.'

'At least one.'

'Still, you know how it would work.'

'At least in theory. I've never used one.'

'I want you to repair it.'

'I want want to repair it,' said the Doctor impatiently. 'Every time you use it incorrectly, the process destabilises Time..' to repair it,' said the Doctor impatiently. 'Every time you use it incorrectly, the process destabilises Time..'

'What do you mean?'

'It's a bit complicated, but trust me, it's not good.'

Chiltern looked down at the slide in his hand. He was silent for a moment. 'So there are other worlds,' he said softly. 'Are they better than this one?'

The Doctor shook his head. 'What you call "the human condition" is universal.'

'Undeserved suffering.'

'I'm afraid so.'

Chiltern nodded, as if this didn't surprise him. He regarded the Doctor with neutral, clinical eyes. 'I'm going to take a sample of brain tissue now, just a few cells.'

'Think again,' snapped the Doctor. 'At least if you want my help.'

'There are ways '

' of forcing me. Yes, yes, I've been through this before. I'd be very surprised if you were ingenious enough to think up something I haven't had tried on me. And I don't force.'

'I could kill you.'

'Oh that would leave you in a nice situation, wouldn't it?'

'You want to repair the machine too,' Chiltern pointed out. 'Possibly even more than I do.'

'I don't think so. I have no personal stake in the matter. This is a silly conversation. If you want to kill me, kill me. I can't stop you. I daresay you intend to eventually anyway. You can always take the brain tissue then. Not that it will help you.'

'Help me?'

'Why are you an alienist, Chiltern? Why did you even want a time machine? What do you think is in the future? An organic solution to madness? Do you think if you sliced me open, you'd find the mind-brain connection sitting there in my inhuman brain, maybe with a little sign pointing to it?'

'There must be an organic solution! Someone, someday, must find it!'

'Why do you care? You don't strike me as an altruist.'

'You ask too many questions.'

'And you didn't ask enough, did you? What about the others? What are they like? And what do they think of the way you botched everything through your carelessness and your arrog'