'What?' she said.

'Oh, I'm so sorry,' he said to her softly. 'I really am.' Martha felt her heart hammering in her chest. It was what he normally said when somebody they'd met got killed.

'We're too late?' she said.

The Doctor snapped out of his reverie to look at her. Again she saw the glimmer in his eyes, that quick and sly intelligence. 'Too late?' he said incredulously. 'Nah. It's just we've only been here five minutes and I already know what went wrong! Hate it when that happens. Well, not hate exactly. It bothers me. Brilliant word, "bothers". Like "oblong". People should use it more. Anyway, good puzzle should take an hour to solve at least. Well, with me slightly less. Like that cornfield maze on Milton Nine.'

'You got lost in that for two days,' said Martha.

'Yeah!' grinned the Doctor. 'Wasn't it brilliant? But this!' He waved a hand dismissively at the bank of controls as he turned to the mouthless man. 'Madness!' He turned back to Martha. 'You know what this is?'

Martha scrutinised the levers, dials and switches. She was acutely aware of the mouthless man watching her, and his leather-aproned colleagues still there in the shadows, too. 'Course,' she said, lying through her teeth. 'And it explains why the ship was never found, doesn't it?'

The Doctor gazed at her with the same utter bewilderment as that time she'd tried to explain about MySpace. Then his face lit up. 'Of course!' he said 'Oh, you are brilliant, Martha Jones! Brilliant!' He turned back to the controls and began to inspect the dials and readings with new-found glee.

Keen to maintain the illusion of brilliance, Martha leant in close beside him to inspect the same dials and readings. The display showed complex swirls and flourishes instead of numbers she could read.

'I think the TARDIS must have crashed quite hard,' she said. 'It doesn't translate this for me.'

The Doctor looked at her over the top of his glasses. 'Nah,' he said. 'They're not numbers as you understand them. They're expressions of atemporal mismatch. Kodicek Scale, I think.' A thought struck him. 'Are you sure you understand how this drive works?'

She shrugged. 'A bit.'

'Right,' said the Doctor. He stood up straight again, stepping away from the controls and stretching his long arms and back. He seemed about to address the mouthless man, then changed his mind and turned back to Martha. 'What bit do you understand?'

'Well,' said Martha. 'It drives the spaceship, doesn't it?'

'Aaaah,' said the Doctor, wagging a finger at her. 'But it's not a spaceship, is it?'

'Sorry. It drives the starship. You can be such a geek.'

'Well,' he huffed, pulling a sulky face. 'These details are important. This drive here means it doesn't travel through space.'

'What?'

'See?' he said to the mouthless man. 'She was really just winging it! Unbelievable these people. And you know what they did to the Dodo?'

The mouthless man stared at him, either not getting the joke or too wary to show that he did.

'Doctor,' said Martha levelly. 'Why don't you tell us what this drive does.'

'Yeah, good idea,' he said. 'What we've got here is really very clever. And a good century ahead of its time. They should be on plain old hyperspace wossnames. But this? It's . . . it's . . .' he twirled a hand in the air, as if it might help conjure the right word.

'It's brilliant?' suggested Martha. Everything was brilliant with him. That's why she'd found a starship called 'Brilliant' so funny in the first place.

'Yeah,' said the Doctor, nodding. 'It's that, too. Cuts out all the boring stuff of travelling between the stars. And there's a lot of boring stuff out there. Billions and millions of miles of it. And empty, mostly, except for background radiation and lots of old TV. There's not a lot to do on the journey to another star. You get old, you die and you just hope your great-great-great-great-great grandkids still remember how to fly the ship.'

'Sounds fun,' said Martha.

'Oh, you lot do it with your usual pig-headed determination to do anything that's completely bonkers. Have I said how you're my very favourite species? But, bit of thinking, and there are ways of cutting corners.'

'Like the Time Vortex,' said Martha, who had taken some elementary lessons in how the TARDIS worked.

'Well, yeah,' the Doctor acknowledged. 'But this lot haven't got anywhere near that far yet. Which is just as well, 'cos I'd be duty-bound to stop 'em. What they've done here is to push against the surface on the outside of the Vortex. It's tough stuff, so it resists and you sort of bounce back off it. And if you can get the angle right – not that you have angles as such in nine-dimensional space – you skip along it, bump-bump-bump. I suppose it's not that graceful, now I come to think about it.'

'So it's like skimming a stone across the surface of a lake,' said Martha.

'Er, yeah,' said the Doctor. 'I wish I'd thought of putting it like that. Can we just pretend I did?'

'Yeah, whatever you like,' said Martha. 'So how does this explain how the Brilliant disappeared?'

'Well,' said the Doctor. 'While all the posh passengers are upstairs sipping cocktails, the ship is lurching across the surface of the space-time continuum like a stone skimming across a lake.' He beamed. 'That is a good analogy! And every time it presses itself into that surface, and just before it bounces back out... Well, it technically skips out of space and time. That's what makes it move so quickly, it misses out most of the actual distance. To anyone looking at it in just four or five dimensions, it's like it blinks out of existence.' He tried to click his fingers to demonstrate, but couldn't make them click. 'You get the idea.'

'Right,' said Martha. 'So the drive makes it flick in and out of reality, yeah?'

'Pretty much,' said the Doctor. 'Now you see me, now you don't. Now you see me again, now you don't again.'

'So it didn't blow up or fall into a black hole,' said Martha. 'It just got stuck somewhere nobody could see it.'

'Oh, I'd have been able to see it,' said the Doctor. 'If I'd gone looking.'

'Well you've got special powers, haven't you, oh mighty Last of the Time Lords?'

'Do I go on and on about that?'

Martha fluttered her eyelashes, all innocence. 'I don't think I've ever heard you mention it.'

'That's OK then. Still, it would have been a bit easier for everyone else to find it if they hadn't kept this technology quiet. I mean, I didn't know anything about this drive. Me!'

'You said there's about to be a big war, didn't you?' said Martha. 'Maybe they wanted to keep it secret from their enemies.'

'Maybe,' said the Doctor, glancing round. Martha realised he didn't want to say whatever it was he really thought while the mouthless man was still listening. But she had an idea herself; the starship's rich passengers weren't just on some wild pleasure cruise. While the rest of the galaxy was struggling not to have a war, this lot had built themselves a clever new way of escaping all the trouble.

Like she and the Doctor would be doing, if they just left in the TARDIS now. She felt awful about that, with the mouthless man stood there. They would be leaving him to his doom.

'Isn't there anything we can do for them?' she asked the Doctor quietly. 'They're going to be lost for ever, aren't they?'

The Doctor took her hand. 'You know how this works,' he said kindly. 'We can't change anything. We have to be responsible. What happens has already happened.'

'Yeah, I know,' she said. 'Still...'

'They also brought this on themselves,' said the Doctor gently. 'This drive is experimental. And they've got staff to run it who can't even tell them when it goes wrong! Oh, that's all very clever for keeping it secret, but it's also pretty stupid.' He turned to the mouthless man. 'No offence.'