'And Captain Lawrence and Stoker,' said Tegan.

'Yes, all of them,' the Doctor agreed. 'But also the people of Akoshemon. An innocent world corrupted and brutalised by the Dark's presence from its very beginnings. All the people we met in the last couple of days, and who lost their lives, amount to little more than that - ' that - ' the Doctor clicked his fingers - 'compared to the billions of lives twisted and ruined by evil on that planet.' the Doctor clicked his fingers - 'compared to the billions of lives twisted and ruined by evil on that planet.'

The Doctor's cold tone provoked an uncomfortable pause. For a few seconds his companions watched the Time Lord resetting the TARDIS co-ordinates. 'What are you thinking?' asked Nyssa.

'That I could use the TARDIS to travel back in time, far enough back in time to prevent the Dark merging with Akoshemon during its creation. Avert the death and suffering of an otherwise doomed world. And thus, perhaps, eventually save the lives of Stoker and her mining team and Captain Lawrence and his crew.'

Tegan said, 'You know you can't do that. It's impossible.'

The Doctor tapped the console irritably 'I know. I know.

But sometimes it's very hard to understand why. why. There are so many things that can be altered, put right. Made better.' There are so many things that can be altered, put right. Made better.'

'You're thinking of Adric again,' Nyssa said.

The Doctor looked up sharply but didn't reply. He didn't need to say anything, because the answer was plain to see in his troubled gaze.' I'm always thinking of Adric.' always thinking of Adric.'

'What's done is done, Doctor,' said Tegan. 'That's what my Aunt Vanessa always used to say. Just because we can materialise before or after "what's done" doesn't make any difference.'

The Doctor frowned slightly as he thought this through and then managed a faint smile. 'Do you know, Tegan, I think you've just managed to summarise all the Laws of Time put together.'

Nyssa couldn't let this pa.s.s. Rather more primly than she intended, she said, 'Surely it's not really as simple as all that!'

'No,' agreed the Doctor. 'But it will do us for now, don't you think?' He dematerialised the TARDIS and the gla.s.s column at the centre of the console began its soothing motion. And, almost as if the ship's transition into the s.p.a.ce-time vortex erased all that had gone before, the Doctor's face cleared and his tone lightened. He clapped his hands together decisively. 'Now, I was thinking.' it's high time both of you learnt to read some of the TARDIS star charts...'

'Not just yet, Doctor,' protested Tegan. 'I don't know about Nyssa, but I'm beat. I've got to get some sleep.'

'Yes,' agreed Nyssa. 'I think we could all do with a rest.'

The Doctor looked disappointed. 'Well, all right then. In the morning, perhaps.'

They nodded gratefully and left the console room. The Doctor stayed behind, leaning on the console and staring into the bright lights of the time rotor, where the past and future mixed to form endless possible presents.

Exhausted, Tegan and Nyssa retired to their quarters and fell into a dreamless slumber.

Both slept with the lights on.