Then the doors closed behind him.

'I can't even see a water hole,' said Jo. 'There should be one around here somewhere. There's plenty of game.'

Rowenna glanced up, saw her friend standing on an outcrop of rock in a classic explorer pose, gazing into the distance, her eyes shielded by a hand. From her own position propped up against the dead trunk of a thorn tree she could see several large, golden antelope warily nibbling at bushes no more than a hundred yards away, and a few dark shapes that might be jackals squatting in the grass.

The alien tree towered over the plain, its colours still flowing in the sunlight. Rowenna wished they could get further away from it.

In the distance, a lion roared. The antelope looked up, their bodies visibly quivering.

Although she knew there was no immediate danger, Rowenna felt her own body tense, felt the sweat trickling down her back. The whole of her back hurt, as far down as it could hurt, as far as the nerves still worked. She wanted to reach around and massage it, but she knew that it was futile. She would just fall over.

'You need to get up higher,' she said to Jo. 'The water hole could be ten miles away.'

Jo stepped down from the rock, sat down in the short grass at its base. Her face was flushed, and there were sweat patches on her shirt. 'I hope it's nearer than that,' she said seriously. 'Julie needs water soon.'

They both glanced over to the long grass near the stark form of the strange tree, where Julie sat with the gun, on guard over Jacob Hynes.

'How sick is she?' asked Rowenna.

Jo shrugged. 'I don't know. She thinks she's dying, but that's just what Hynes says. She's certainly got a fever. Are you OK?'

Rowenna hesitated, then decided on the truth. Lying in this situation wasn't brave: it was just stupid. 'I think I'm a bit feverish. My throat's dry. But my back's hurting a lot, so it's hard to tell. I might just be shaken up.' She hesitated. 'What did Hynes say?'

'Just some demented apocalyptic nonsense. He's going to cleanse the earth of humans, that kind of thing. I don't think he knows where or when we are any more than I do. Though he did admit that the tree is capable of time travel.'

'Time travel?' asked Rowenna. Rowenna looked at the alien thing. 'Are you sure? I mean, I know... I assume that "horse"

wasn't from Earth, but '

Jo cut her off. 'It was night when we left the cave, it was midday when we got here. I only noticed a few seconds passing. So we moved half a day, at least. Probably much further there don't seem to be many signs of civilisation around.'

She stood up again. 'Look, I'm going to have to try to find some water. Or a village, or something. And ' She broke off, staring over Rowenna's shoulder.

Rowenna turned awkwardly to follow her gaze, keeping her balance with one hand planted firmly in the hot dry earth.

For a moment she saw nothing.

Then she realised that there was someone standing in the grass.

Several someones. With rough brown hair, shaggy manes, and angry brown eyes.

All of the animals were male. And all of them were about five feet tall and built like all-in wrestlers. One of them grunted, and began to beat his fists against his chest, tossing his head from side to side so that the rust-coloured mane flew in the wind.

Rowenna swallowed. This was impossible. This was impossible. 'Did you say time travel?' 'Did you say time travel?'

Jo nodded. 'Let me guess. This lot have been extinct quite a while.'

Rowenna looked at the apelike faces, the almost human posture of the animals. 'If I'm right, they're australopithecines,'

she said. 'I don't think you'll find that village, Jo.'

Jo turned to her. 'How long have they been extinct?'

Rowenna swallowed again, still struggling to come to terms with the implications of what she was about to say. 'Two and a half million years.'

CHAPTER 13.

Sam punched the TARDIS door control furiously, but nothing happened. She hadn't expected it to.

After a moment she heard Kitig's heavy footsteps approaching. 'This is a time machine, isn't it?' asked the Tractite.

Sam nodded miserably.

'And the Doctor is going to change the history of our world, so that the Tractites don't exist, just as you "hypothesised"

on the skimmer?'

Sam nodded again. She felt like crying.

She felt a three-fingered hand touch her arm gently. 'You are not responsible,' said Kitig. 'The Doctor is. And I am.'

Sam turned round, found herself looking right into Kitig's eyes: the Tractite had knelt down so as to be level with her.

'You're responsible?'

She listened as Kitig explained the role of the Keeper in the Tractite world: to watch for the coming of someone who might never come, who might be no more than legend but who might destroy everything.

'And there's one in every city on Earth?' she asked when he'd finished.

'Every city in the galaxy,' said Kitig quietly. 'But you must understand, our role has changed over the centuries. The main threat is that innocent travellers who merely happened to resemble the description in the book would be destroyed by other Tractites, in case they were the Uncreator. I was given the job of Keeper by my father because I showed qualities such as calm judgement, rationality under pressure, and mercy towards others. He and I never really believed that there was an Uncreator.'

'And now?'

Kitig let go of her arm, sat down on the floor, folding his legs beneath him. 'It seems that we were wrong,' he said after a while.

Sam took a breath. 'Look, Kitig, you've got to fight fight. You've got to do something. You can't just give in like this.'

'I'm not giving in,' said Kitig. 'But there's nothing I can do, is there?'

Sam thought about it. There had to be a way. She couldn't let Kitig out of the TARDIS, but she was pretty sure there would be a way that she could get out. She knew the Doctor. He always left a back door open.

She looked towards the carved entrance to the cloister room. 'I might be able to help you,' she said slowly. 'I could go after the Doctor I could at least talk to him, try to find some other way.'

'I can't ask for your help, Sam. Your loyalty must lie with your own people.'

Sam swallowed. 'I think your people are better. I saw what our people did to them, when in the other universe, or wherever it was.'

'I know. That story may or may not be true. Your people may or may not be "worse" than mine, if you can make such comparisons about an entire species. But if you helped to destroy humanity or prevent their creation you would be betraying all your life up to the present time. Your parents, your siblings and friends, your children '

Sam had to laugh. 'I haven't got any children! I'm only seventeen!'

Kitig snorted softly. 'That may explain it. Imagine, then, how your parents would feel, if they were in your position and were asked to betray you to save another species.'

Sam thought about it.

Her mother. Her father. Would they think the Tractites were better than humans?

Yes. Almost certainly.

Would they abandon her in order to save the Tractites?

Well, they'd abandoned her to save the whale. They'd gone on marches, weekend protests, left her with Gran or with friends. Or made her march with them back when she hadn't wanted to, hadn't really understood what it was all about.

But would they leave her to die?

She thought of her mother's earnest face, her father's grey beard and old blue rollneck sweater.

No. They'd marched as an investment in the future. Her future. For her. And they were trying to save, not destroy things.

Nonetheless, they would be on the Doctor's side, if they were here instead of her.

And they would be doing it because because of her. of her.

Sam shook her head, reminded herself to make sure that the Doctor dropped her off soon at some point in time not too long after she'd vanished, so that she could tell them she was OK, and not to worry. Or at least send them a postcard.

Except that Unless the Doctor succeeded, they'd already ceased to exist.

'Oh,' she said aloud, and sat down rather suddenly.

Kitig looked at her.

'My parents don't exist any more unless I ' She broke off.

'You see?' said Kitig gently. 'It's not so easy, making choices.'

Sam stared up for a long time at the brass infinity of the console-room sky. Finally, she stood up.

'Kitig,' she said. 'If I go after the Doctor, will you promise me something? Will you promise me that you won't try to come after me that you'll stay here?'

Kitig looked at the ground for a moment, then met her eyes. 'I promise.'

Sam was certain, absolutely certain certain, that she could trust him. It was the only thing she was certain about at the moment.

She walked around the console, towards the huge, dark, door that led to the cloister room.

'So you're saying this is two million years ago?' Julie was shivering despite the heat: she was very obviously feverish, her skin pale, sweat dripping off her face. She'd given the gun to Jo while she worked on tying Hynes's hands together with shredded pieces of his shirt.

Jo shrugged. 'Time travel isn't all that rare. The Doctor's lot try to keep it to themselves, but you can't really stop a technology from spreading. I've never seen a tree do it, but I dare say the Doctor could explain it.'

'Who's the Doctor?' asked Hynes suddenly. His voice was thick with spit and fear, but Jo had no sympathy for him. Julie was ill. Rowenna was ill. Probably they were dying. If Captain Hynes was telling the truth about infecting them with a virus, he was a murderer.

'But how are we going to get back?'

Jo knew that Julie was still thinking about a hospital, about treatment for whatever nightmare disease Hynes's alien friends had given her. But Jo had seen too often what happened to people infected by alien viruses: she didn't have much confidence in the ability of twentieth-century hospitals to save Julie's life.

'If only the Doctor would turn up,' she muttered aloud. 'But then, timely rescues never were his strongest point.'

Julie's eyes fixed on hers. 'D'you reckon he might turn up, then? This friend of yours? Could he have an antidote for this this thing?' Her voice was cracked, desperate.

'I've no real reason to believe he's anywhere near this place,' said Jo carefully. 'But...' She shrugged again. 'Call it a hunch.'

Julie sat back suddenly, swaying on her knees.

Jo stepped forward, and saw Hynes move at the same time. 'You keep still,' she cautioned him. 'Julie?'

Julie had put both hands on the ground to steady herself. She looked at Jo with terrified eyes. 'You'd better give Rowenna the gun,' she whispered. 'I'm losing it.'

Jo nodded. She checked the knots holding Hynes, and decided that he was reasonably secure. Jo closed her eyes for a moment, suddenly aware that she was thirsty. Very, very very thirsty. Had she been infected too? thirsty. Had she been infected too?

She walked slowly over to the thorn tree where Rowenna was sitting.

'I guess I'm pretty nearly useless here,' said her friend as Jo approached. Her voice was distinctly strained: Jo supposed that she was in more pain than she was owning up to.

Jo squatted down next to her under the tree and put an arm round her shoulders. 'I feel pretty useless myself,' she confessed. 'We'll just have to do what we can. Hope that the Doctor can find us. Or that we can somehow make the time tree go back.'

Rowenna's shoulders hunched under Jo's arm, hard bone and muscles shivering with tension. 'I don't think we should rely on either of those things. I mean, if the time tree dumped us here, it could dump us anywhere next time. We don't even know how to work it. And as for your friend, you haven't actually seen him for twenty years, have you?'

Jo shook her head. 'I just don't know what else we can do,' she admitted.

'Find our ancestors,' said Rowenna quietly after a moment.

Jo frowned. 'I thought we'd found them.'

Rowenna shook her head. 'No, no. Not the australos. Homo habilis. Homo habilis. Our ancestors in the direct line. They'll be a lot smarter. They'll have access to water. Maybe they'll look after us if we get sick, I don't know. We'll survive longer if we can get them to help us.' Our ancestors in the direct line. They'll be a lot smarter. They'll have access to water. Maybe they'll look after us if we get sick, I don't know. We'll survive longer if we can get them to help us.'

Jo stared at her.

'It's not impossible,' said Rowenna. 'Anthropologists have often made good relationships with ape communities. These people are a lot closer to us than apes.'