And then she realised she was being watched.

There were six of them, short, stocky men wearing tough leather aprons and luridly coloured Bermuda shorts. Practical, she thought, for this hot and heavy environment. They lurked in the shadows by the machines, watching her and the Doctor nervously.

'Er, hi!' she waved at them. One of them waved back instinctively then hid his hand behind his back. The men remained where they were, skulking in the darkness.

They're more scared of us than we are of them,' said the Doctor quietly.

'You said that in Kenya about those lions,' said Martha.

'Well, yes,' admitted the Doctor. He smiled his brightest smile as he addressed the men. 'She's feeling a lot better now, thank you. Said a tea would do the trick!'

The men remained in the shadows, watching. The Doctor nudged Martha in the ribs with a bony elbow.

'Come on,' he said, stepping forward. 'You need to thank them for the tea.'

'Right,' said Martha, feeling awkward. She hated being pushed in front of people, expected to perform. Her mum would still have important workmates round for dinner sometimes. Tish and Martha were always made to hand out the nibbles – her brother Leo always got away with filling up people's drinks. This is my middle one,' Mum would preen as her friends took the stuffed olives or carrot sticks and dip. 'She's going to be a leading surgeon.' It always made Martha furious, but she had never answered back. Tish, who liked playing up to her mum's image of her, said Martha had a twisted sense of duty. And Martha knew she was right. Even now, hundreds of years in the future, she felt herself adopting a familiar, joyless smile.

'Hi!' she said with badly faked delight. 'I'm Martha!' The men in the leather aprons said nothing and remained where they were. She turned to the Doctor. 'You did introduce us, didn't you?'

'Er,' said the Doctor sheepishly. 'I did call out a bit, but nobody responded. They probably didn't hear me over the noise of the engines. And then I found the kitchen and sort of helped myself. Sorry! Better leave them some coins in case they've got a tea club!' He rummaged through the pockets of his suit jacket, the inside ones first. 'Can you remember what the money is in space in the fortieth century?' Martha felt guilty; only a couple of days before she'd thrown a gold sovereign away down a wishing well.

The men in aprons seemed to cower in the darkness, and Martha realised they must think the Doctor was looking for a weapon. The poor blokes were terrified of them and she started to understand maybe why. They were the lowest of the low, toiling away in this noisy, sweaty place. They would never mix with any of the ship's passengers, and they probably only ever heard from the crew when something had gone wrong.

She reached a hand into the inside pocket of the Doctor's jacket, helping herself to the slim leather wallet that he kept with his sonic screwdriver. He raised an eyebrow at her but otherwise didn't seem to object; he liked it when she showed some initiative.

Martha flicked the wallet open, paused to picture in her mind what she wanted it to show, and then brandished it at the men still lurking in the shadows.

'There's no need for any concern,' she said, adopting the confident, reassuring tone that she'd learned from Mr Stoker. 'We're not here on an inspection. My assistant here—' she nodded her head at the Doctor'—just needs to familiarise himself with the ship's workings as part of his training. We'll just be a couple of minutes and then let you get back to your work.' She smiled her most charming smile.

The men in the Bermuda shorts and aprons turned to each other, said nothing yet seemed to confer.

'That was good,' said the Doctor quietly, taking the wallet of psychic paper from her and pocketing it carefully. Slowly, one of the men in aprons shuffled forward, glancing back to his friends, who all kept safely where they were. Martha's heart went out to the poor bloke. She thought he might have been the one who had waved before.

'That's it,' she told him. 'Me and him, we're really nothing to worry about. I'm Martha, he's the Doctor. Who are—'

The sentence died in her throat as the man in the leather apron stepped out into the light. He was tall and muscular, his eyes alive with fear and excitement. And he didn't have a mouth.

TWO

Martha realised she was staring, her own mouth hanging open. The man in the leather apron and the garish Bermuda shorts stared back at her mutely. Below the man's nose, where a mouth should have been, there was just a small, round hole, the same size as if it had been made by a hole-punch. His glistening black stubble didn't divide into beard and moustache, but covered the lower part of his face evenly. 'Right,' she said, not sure what she would say next. 'Right,' she said again.

'I think what my superior is trying to articulate,' said the Doctor, nimbly taking charge, 'is that we're very keen not to disturb what you're doing. We'll just keep out of your way.'

But the mouthless man raised his fist and began gesturing wildly. Martha grabbed the Doctor's arm to pull him back, worried he might get himself hit. The Doctor shrugged her off, and began to wave his own arm in a similarly emphatic manner.

It was some rudimentary kind of sign language.

'What's he saying?' she asked.

The Doctor and the mouthless man continued to wave their arms at each other. 'I think,' said the Doctor, 'he wants us to go that way.' He stopped waving, and pointed in the direction that the mouthless man was still indicating. The mouthless man nodded vigorously. 'Yes, I think that's what he wants.'

'Sorry,' said Martha to the mouthless man. 'But you can understand us, can't you? You can't speak but you understand English?'

The mouthless man nodded, then looked back at his colleagues. In the shadows, they nodded too. 'Oh,' said the Doctor. 'That's a good point. So, whoever you answer to, whoever gives you orders, they can tell you what to do out loud.'

Again the mouthless man nodded, and Martha felt a thrill of fear. It wasn't that this was a new species of people who just weren't born with mouths. Instead they were some kind of lower order of men, able to take instructions yet not to answer back. Either they'd been bred like this or they'd been operated on, but whatever it was they were clearly some kind of class of slaves.

Martha could see in the Doctor's eyes the same determination she felt burning hot inside herself. Whatever happened, they were going to help free these people.

The mouthless man gestured again down the passageway, beckoning the Doctor and Martha to follow him. They continued up the alley between the huge machines. The mouthless man's bare back showed strong shoulders and toned muscles, Martha noticed. His Bermuda shorts were all swirls of pink and blue.

The alley emerged into a wide, open area, about the size of Martha's tiny flat in London. The far wall was covered over with a bank of complex levers and controls. Not needing to be prompted by the mouthless man, the Doctor put on his glasses as he hurried over to inspect it.

Martha, knowing she'd make nothing of the controls herself but keen to at least look interested, headed over to a small, inset porthole to the left of all the switches. It must be some kind of inspection hatch for looking into the machine, she thought. She gazed in on a pale grey light that swirled gloopily beyond. Despite the clammy heat of the engine room she found her bare arms suddenly prickling with goose bumps. There was something scarily familiar about that grey light, but she couldn't think what it could be.

She turned to the Doctor to ask him. His mouth hung open and there was a mixture of awe and horror in his eyes.