'But won't they get bored with this song one day?' she asked the Doctor.

'A-ha!' he said brightly, producing a yo-yo from his pocket. 'No, hang on, sorry.' He handed the yo-yo to her and had another go. Almost. Don't worry, I've done this before.' And he produced the innocuous-looking key. 'Yes they'll get bored,' he said as he unlocked the door to his spaceship. 'But they were programmed as holiday reps, weren't they? Every one of them's a born entertainer. They've got hooks and beats in their chips.'

Martha gaped at him. 'They'll make their own music, won't they?' she laughed. 'They'll entertain themselves.'

'Right on, sister,' grinned the Doctor. 'A bit of culture to liberate the workers. Come on, let's leave them to it.'

A moment later, with a gruff rasping, grating sound that tore through the fabric of time and space itself, the police box was gone from the alleyway. Six thousand robots lived happily ever after.

'So where next?' said the Doctor, fussing with the TARDIS controls. His long, skinny fingers danced across the strange array of instruments and dials, his face lit by the eerie pale glow from the central column.

'What about that spaceship?' said Martha.

'That spaceship,' agreed the Doctor. He began to set the coordinates, then stopped to look back up at her. 'Which spaceship?'

'That spaceship you were telling me about. When we were waiting to be executed.' She sighed and rolled her eyes. 'Just a minute ago!'

The Doctor's eyes narrowed to slits as he struggled to remember. 'Oh! That spaceship,' he said after a moment.

'Come on' she said, 'you said it was brilliant.'

'Well it was. Literally. The Starship Brilliant. Luxury passenger thing. In space. But I only told you about it to take your mind off, well, you know...' He drew a finger quickly across his neck.

'Yeah, but come on,' said Martha, leaning towards him across the console. 'You said nobody knew what happened to it. Not even you.'

'Well no,' he said, scratching at the back of his head. 'Not exactly. I mean, there are theories.' He began to step lightly around the control console, flicking switches, careful not to meet her gaze. 'It could have fallen into a black hole, or crashed into a giant space squid. You know it vanished just before a huge galactic war?'

'No,' said Martha.

'Well. That could mean something couldn't it?'

'Oh come on,' said Martha, 'you know you want to. It's a mystery!'

'Yeah, well.' The Doctor thrust a hand into the trouser pocket of his skinny, pinstriped suit; his way of looking casual. 'Exploring a spaceship that you know is going to vanish forever... Probably be a bit dangerous. Dangerous and reckless. Dangerous and reckless and irresponsible.'

'What?' she laughed. 'And never know what happened to it? Ever? That's not like you at all.'

The Doctor gazed at her, deep brown eyes open wide. Martha felt the smile on her own face falter, her insides turning over. She had come to accept that the Doctor didn't share her feelings for him, but sometimes the way he looked at her...

'So we're going?' she said quickly.

'It'll bother me if we don't,' he said, busy now with coordinates and the helmic regulator. He stopped to look back up at her. 'But there are some rules. Important ones.'

'Whatever you say'

'Yes, whatever I say.' Martha did her best to look serious. 'One,' the Doctor continued. 'We can't get involved with anyone we meet. Two, we absolutely cannot change anything. Not a bean. Nuffink. Nada. Nana nee-nee noo-noo.'

'Right.'

'And three...' He turned from the controls to look at her and his eyes sparkled as he grinned. 'Oh, what's the use?' he said, and plunged the lever to send them hurtling back in time.

'Honestly, it'll be fine—' began Martha.

But the huge explosion cut her sentence short. She was thrown off her feet, hurled head over heels across the TARDIS console to crash hard into the metal mesh floor.

Typical, she thought, as everything faded to black.

ONE

In the moment after she woke and before she opened her eyes, Martha thought she was in her mum's house in London. She could smell strong tea and cleanliness all around her as she lay sprawled on her back. Her jeans and leather jacket dug into her skin, she felt hot and heady like she'd had a late night out and the floor was trembling beneath her. Sore and a little bit fragile, she dared to look around.

Dark. Industrial. Noisy. Not the TARDIS. She closed her eyes again.

When she next awoke, she found the Doctor crouched beside her, grinning encouragingly. He brandished a chipped china mug at her with a drawing of a sheep on it.

'A little milk and no sugar, yeah?' he said.

'Ta,' she said, struggling to sit up. Her head throbbed and her limbs felt shaky, so she checked herself over for concussion. She wiggled her fingers and toes, and closed one eye and then the other to make sure her vision was OK. Everything seemed to be fine. Martha could remember the explosion in the TARDIS, being thrown off her feet and across the console, so she wasn't missing any memory. And, for all she felt battered, she didn't feel queasy, so there didn't seem to be any internal damage to worry about.

'What's the diagnosis?' asked the Doctor, with that slight, admiring smile he kept for whenever she showed a bit more intelligence than your average human ape.

'OK, I think,' she said. 'Can you check my pupils?'

He handed her the mug of tea and fished in his pocket for his sonic screwdriver. Its brilliant blue light dazzled her for a second. 'Both the same size,' said the Doctor. 'Both go all small when I shine a light at them. That's what they're meant to do, isn't it?'

'Means I'm probably not bleeding to death on the inside,' she said, batting the sonic screwdriver out of her face. 'I'm happy with that.'

So she had survived intact. And then she realised it was not a headache she could feel but the deep bass line of vast machinery thrumming all around her. They were no longer in the TARDIS. Wherever they were it stank like washing-up liquid, all efficient and clean. And it wasn't her own body that was shaking; the hard metal floor beneath her trembled with terrible power.

Martha drank the strong and pungent tea while glancing round to get her bearings. They were in the narrow alleyway between two huge machines; huge and noisy as an old factory or printing press, she thought, a whole series of sturdy great machines working flat out. She was suddenly reminded of the dark, low-ceilinged basement at the Royal Hope, where the hospital had its own power generator. Her mate Rachel had taken her down there at the end of a night shift to watch some other medical students lose at cards to the porters. Martha remembered them squeezing into a small, sweaty, claustrophobic room where you couldn't even hear yourself think. This place had the same heavy, oppressive feel to it.

'We hit the engine rooms then?' she said.

The Doctor grinned at her. "Very good,' he said. 'Yeah, smacked right into it. Sorry. Think they must have some sort of unmentored warp core or something, and the TARDIS went a bit rabbit-in-the-headlights. Doesn't take much to turn her head these days, poor girl. I meant to put us down in the passenger lounge. Bet it's a lot more posh than this upstairs.'

'Right,' said Martha. She put down her tea and struggled unsteadily to her feet. Just along the alleyway stood the reassuring shape of the TARDIS. She could still taste the acrid smoke that had billowed from the console, and realised the Doctor must have carried her out of it, letting her down here before hurrying off to find help . . . and the mug of tea. The engines around her filled her head with noise and her skin felt itchy with grime. Yet the dark and solid machinery seemed immaculate; perhaps she was just imagining the dirt. She shrugged off her jacket, the air suddenly hot and clammy on her bare arms. Despite the heat, she shivered; there was something wrong about this place, she could feel it deep inside her.