Tegan was happy to have a few minutes to herself. Several parcels had been delivered from Harrods, the fruits of her shopping expedition that morning. Not noted for her patience, Tegan was keen to unpack them right away.

She waved to the Doctor as he unlocked the door to his room. But he seemed not to notice. She could see that he was already deep in thought.

He pulled a battered notebook from his jacket pocket, and went into his room.

By half-past ten Tegan was fed up with waiting. She paused for the briefest of moments outside the door to the Doctor's room, then knocked. There was no answer, so she opened it and went in.

The Doctor was lying on the bed, hands clasped behind his head, staring at the ceiling. His notebook was still open, but laid face down on his chest.

'Ah, Tegan,' he said without moving, 'do come in.'

'You're late,' she said. 'And I'm hungry.'

'Dinner,' the Doctor heaved himself off the bed. 'Oh yes.' The notebook fell to the floor, and he picked it up.

'Any new clues?' Tegan asked as she followed the Doctor from the room.

'Mmmm,' he said. 'I'll tell you after we've eaten.'

'Don't want to spoil my appet.i.te?' she joked.

The Doctor glared at her, then set off down the corridor without answering.

The dining room was almost empty. An old man sat on his own at a table near the door. A middle-aged couple occupied a booth in the far corner.

The old man eyed the Doctor and Tegan suspiciously as they waited to be seated. The Doctor smiled at him and Tegan frowned.

'Colonel Finklestone,' the man barked suddenly, wiping his mouth on his napkin. 'Don't have the salmon.' The waiter scowled at the man as he arrived to attend to the Doctor and Tegan.

'Er, thank you,' the Doctor replied. 'The Doctor, and Miss Tegan Jovanka.'

Colonel Finklestone snorted as if they had in some way insulted him, and returned his attention to his wine.

'Doctor, Miss Jovanka,' the waiter smiled widely and nodded to them, picking up on their names, 'dinner for two?'

'Please,' the Doctor replied. 'A table near the window perhaps?'

'Of course, sir.' The waiter led them across the near-deserted room. 'Will this do?' he asked as they reached the table where they had eaten breakfast what seemed like several months previously.

'Admirably, thank you.' The Doctor seated himself and accepted a menu and wine list. The waiter pulled back Tegan's chair for her as she sat down.

'Oh no you don't,' Tegan said before he could push the chair in again, dragging it in closer to the table.

The waiter left them to look at the menu. Tegan flicked through, remembering the brief conversation they had exchanged before.

'I believe I'll have the oysters,' the Doctor said, laying his menu to one side and picking up the wine list. It was leather-bound, with a gold cord down the spine ending in a ta.s.sel.

'You know you will,' Tegan said.

'Yes, but you have to go through the motions.'

'Why?' Tegan dropped her menu heavily on the table by her plate. It clattered against the lead crystal and disturbed the double damask. 'You keep going on about how we can't change things, but you won't prove it.'

'I don't need to. I know.' Tegan looked out of the window. The moon was shining through the murky night, its light diffused across the surface of the Thames outside. Snow was falling lazily through the smog, spiralling its way through the young trees which edged the Embankment.

'Doctor,' Tegan said quietly, 'in about an hour, we will arrive at the British Museum. What's to stop us - this us - going there and warning us - that us - to leave before anything happens to Nyssa?'

The Doctor said nothing for a while. He stared out of the window, or perhaps he was watching Tegan's reflection in the gla.s.s as she continued to watch the snow.

'You see those snowflakes,' he said at last.

Tegan nodded.

'As they twist and tumble their way down, they collide with each other, get swept away in the breeze, melt in a warm updraft. Now, imagine you plotted the course of one of those snowflakes, and you found that it collided with another snowflake. And you found it collided with it not just once, but twice.'

'So?'

'And then you changed the course of one of the snowflakes so that the first collision never happened.'

'Yes. So what?'

'So, would the second collision happen?'

Tegan considered. 'Maybe. You can't tell.'

The Doctor nodded. 'That's right. As soon as you change any of the circ.u.mstances, all bets are off. The second collision may occur, or it may not. There may be a completely different collision, or the snowflake may melt on a gas lamp before it reached its rendezvous.'

'What's that got to do with Nyssa?'

The Doctor beckoned to the waiter who was standing on the far side of the room. 'Everything,' he said. 'If we change things, we have no idea what will happen as a result, what collisions we set up, what courses we alter. We might all end up mummified, and that would help n.o.body. Maybe we should try, but that's not the way Time works. You only get one chance.' He leaned forward and looked deep into Tegan's eyes. 'We couldn't change a thing.

We could go to the Museum, but we'd be delayed on the way, or miss ourselves somehow. Which is probably as well given what would happen to the temporal differential if we actually did meet.'

'How do you know you can't change it? You won't try.' Tegan's frustration was cut short by the arrival of the waiter.

'Oysters,' the Doctor said immediately. 'And a bottle of the Morgon.'

Tegan had not decided, though she knew what she would not be having.

She grabbed back her menu as the waiter lifted it from the table, opened it, and went for the first thing she saw. 'Ham.'

The waiter was still startled. He blinked, then realized that Tegan had ordered. 'Very good,' he said, bowed, and left.

'Not having the cutlets?' the Doctor asked as soon as the waiter was out of earshot.

'Didn't fancy them.'

'And I thought you were making a point.' The Doctor turned his attention back to the snow outside. 'It's a shame you don't fancy the cutlets, though,'

he said quietly, 'since you'll have to eat them.'

Tegan forced herself to keep relatively calm. 'How do you know?' she asked, on hand clenching on the edge of the tablecloth.

'Look, Tegan,' the Doctor cleared aside his cutlery and rested his hands on the table as he leaned forward. 'You want to go and warn ourselves at the British Museum to leave at once.'

'Yes.'

'And what do you think will happen if we do that?'

'We'll leave,' Tegan said. 'We'll go back into the TARDIS and Nyssa will be all right.'

The Doctor nodded. 'And if we had done that, which we didn't, then who will warn us?'

'What?'

'Look,' the Doctor said, pressing his hands together and raising them so that his index fingers almost touched his lips. 'If we had left straight after we arrived, either through some warning or on a whim, we would not be here now. So we could not be having this discussion, coming to a conclusion, or rushing to warn ourselves. The fact that we are here now means that we didn't - will not - leave.'

Tegan frowned. 'So we can't change anything?'

'Well, I have seen it done. But never without immense initial cost, and always so that history returns to its original track as soon as it gets the chance.'

The Doctor leaned back in his seat as the waiter approached again. 'You needn't take my word for it though.'

The Doctor tasted the wine, swilling it noisily round his mouth and smacking his lips together appreciatively. He nodded his approval, and the waiter sloshed some wine into Tegan's gla.s.s, then carefully poured for the Doctor.

'Charming,' Tegan said as he left.

The Doctor smiled. 'An example of what we were just discussing, surely.'

'How so?'

'Why were you rude to him when we arrived just now?'

'Because he was so snotty last time.'

The Doctor nodded. 'But for him, last time hasn't happened yet. Though when it does, at breakfast tomorrow, he'll be snotty to you. And he'll be snotty to you because you were rude to him the night before.'

'Which for me won't have happened yet.'

'Exactly.' The Doctor sipped at his wine. 'This is rather good, you know. It's a sort of self-fulfilling prophecy.'

'I'll be nice to him, then,' Tegan said.

'Well, here's your chance.' The Doctor nodded to the waiter as he approached again, this time pushing a low trolley. 'Good luck.'

Tegan smiled sweetly at the waiter as he removed the silver lid from the platter he presented to the Doctor. He did his best to ignore her. 'Oysters, sir.'

Tegan continued to smile as he produced her dinner, trying to make meaningful eye contact as he set it before her.

'Cutlets, madam,' he said.

Tegan's smile froze. 'What?'

'Cutlets. You asked for lamb.'

'My compliments to the chef,' the Doctor slipped in quickly, 'these oysters are magnificent.'

Then Tegan exploded.

'We need to pack,' the Doctor said as they made their way up the stairs back to their rooms.

'Oh? Why?'

'Two reasons. First, the rooms will be needed later tonight.'

'By us, I remember.'

'And second, before dinner I managed to decipher some of the hieroglyphics I copied down from the tomb.'

They paused outside Tegan's room. 'And what does that mean?' she asked.

'I think it means that reviving Nyssa, starting irreversibly the hundred-year cycle to bring her back to life, was one of my less inspired actions.'