A face looked down at Sam, dark against the blazing noon sky. A human face, grinning broadly.

'You OK?'

Sam sat up slowly, looked around. The creatures were gone. The air smelled of their breath, of monkey sweat.

She closed her eyes for a moment, then opened them and looked at her rescuer. He was a young man, shabbily dressed, with dust on his clothes and a long-bladed knife in his hand.

The knife dripped blood.

Sam stood up, rather surprised that her head didn't hurt any more than it did. 'Thanks,' she said, extending a hand. 'I'm Sam.'

'I'm Jacob,' said the young man, shaking hands. 'Captain Jacob Hynes, of UNIT. I'm here on a special mission.' He stared at the rocks for a moment: Sam followed his gaze, saw a trail of blood on the rocks.

Jacob met her eyes, nodded grimly. 'You're with the Doctor, I suppose?'

Sam hesitated, thought about the Doctor's firm instructions to stay in the TARDIS, her disobedience. 'Sort of.'

Jacob nodded, grinned. 'We're "sort of" working with him, too.' He pulled an odd-looking object out of his pocket: a hypodermic syringe attached to a green sphere about the size of a tennis ball.

The sphere quivered.

'It's a blood sample,' said Jacob. 'We've got to inject it into one of these habilines. If we can catch one, that is.'

Sam frowned. 'Why? Did he say why?'

Jacob hesitated. 'It's sort of classified,' he said after a moment. 'What's your status, exactly?'

Sam grinned. 'I live in the TARDIS.'

A pause. 'Ah right.' He looked away, a strange, tense expression on his face. 'Look,' he said at last. 'I need to follow the wounded one. I'll try and grab him, get this stuff into him like the Doctor said. I'd appreciate your help. So would the Doctor, I'm sure.'

Sam looked at her shoes. She wasn't sure about this man. He was trying a little bit too hard to convince her. But on the other hand, he probably wasn't sure if she was telling the truth, either. And she didn't know what he'd been through to get here.

'Are you coming with me?' Jacob had already started up the slope. The impatience was visible on his face.

Sam hesitated, then followed him. The Doctor had been right: she was going to have to decide. If this man was helping the Doctor to save these what did he call them? these habilines, then she could either let him do it, or prevent him.

And given that straight choice, between saving the human race and not saving it, she knew what she was going to have to do.

She wondered for a moment if Kitig would feel it happen when his world was uncreated.

Jo ran, sweat streaming down her face and legs and sticking her clothes to her body.

She hadn't found any water, only a solitary patch of slightly softer earth and greener grass. She'd been heading for the mountain in search of something better when she'd heard the distant warble of what sounded like a rape alarm. She'd started running straight away, following the easy marker of the huge alien tree, but she'd been at least a mile from Rowenna and Julie. Now she was exhausted: her lungs heaving, her forehead burning. Sweat was dripping into her eyes, and she could barely see in front of her. Ahead, she could make out a big horselike shape that could only be one of Hynes's alien friends, and someone standing next to it Hynes?

'Rowenna!' she yelled, then wondered if it wouldn't have been better to keep her head down.

The figure standing by the alien turned to look at her. It wasn't Hynes, but there was something familiar about him. The shoulder-length hair, the strange fancy-dress costume. She'd seen him before somewhere somewhen 'Jo!'

And she knew then.

'Doctor!'

He ran up to her, took her hands. 'Jo! I'm so glad to see you!'

Jo studied the young man. The unlined face, the bright young eyes, the untidy hair.

Young again.

Lucky Doctor.

'Where are Rowenna and ' she began.

Then she saw Julie's body on the grass. Saw the blood there.

She realised that there was blood on the Doctor's hands. Blood on her hands. Blood 'What happened?' she said weakly, looking up at the alien creature. 'Did he '

'No. He's harmless. They were attacked by dogs.' The Doctor turned away, and Jo could hear the pain in his voice. 'I was too late.'

Jo saw the crushed corpse of a dog on the grass, and another beside Julie's body. There was something bloody and dead and too big to be a dog somewhere beyond the alien, but Jo wasn't going to look, couldn't bring herself to look.

'The responsibility is mine.' An unfamiliar voice. The alien was speaking. 'I delayed the Doctor for reasons of my own.'

Jo sat down in the grass. 'I was going to find them water,' she said hollowly. 'I should have been here.'

'I was was here.' The pain was still in the Doctor's voice, and he was looking away from her, hiding his face. Jo wondered if he was actually crying. He had never used to do that. 'I here.' The pain was still in the Doctor's voice, and he was looking away from her, hiding his face. Jo wondered if he was actually crying. He had never used to do that. 'I promised promised them.' them.'

Jo stood up, walked up to the Doctor, put a hand on his shoulder. 'We've been here before, Doctor,' she reminded him.

He put a hand over hers. There was still blood on it. Jo closed her eyes for a moment. She'd often wondered what it would be like to meet the Doctor again. She'd never thought that it would be like this in the middle of a hot, ancient plain, with two people dead beside them.

'What are the funeral customs of your people?'

Jo looked up, startled. The alien was looking down at Rowenna's body. The three-fingered hands were visibly shaking.

Not like the creature in Hynes's cave then. As the Doctor had said, harmless.

'I don't mean to be offensive,' the alien went on. 'But we should deal with the bodies or they will attract scavengers.

There is no dignity in that.'

'Can you carry them?' asked the Doctor.

Kitig tossed his head slightly: a nod.

'We'll take them to the TARDIS then. When we get back ' He paused. 'If we get back, I can arrange for them to be buried at home. If not, at least...' He shrugged.

'If we get home?' asked Jo gently.

The Doctor turned to her, shook his head. 'There's a problem with the time stream. That thing ' he gestured at the time tree ' and some rogue members of Kitig's species, Tractites, have tried to time-loop the human race. It couldn't possibly work not with a one-way time-travel device so all they've done is destabilise the vortex. Right now everything outside this area is just a quantum flux, a sort of fog of might-be-or-might-not-be. I have to sort it out or ' He swung round suddenly. 'Kitig, how did you get out of the TARDIS?'

'I followed Sam.'

'Sam! I should have known ' He whirled back to face Jo. 'Jo, you've got to find Sam for me. Jacob is still out there you met Jacob Hynes?' Jo's face must have shown him the answer, because he went on without giving her a chance to speak.

'You've got to find her, and you've got to find Jacob, before it's too late.'

'Too late for what?'

'Jacob's trying to wipe out the human race, but that doesn't really matter what matters is the stability of the vortex.'

Jo raised an eyebrow.

'Well, all right, both, then. In fact they're inextricably linked. The human race is simply so important to everything that happens in this sector for the next few thousand ' He broke off, seemed to sniff the air. 'Jo, please please, you've got to help me.

There's more going on than Jacob's little plans, and I've got to find out what it is.'

Jo shook her head. 'I'm not twenty any more, Doctor. I want explanations. Why do you think there's something else going on?'

The Doctor met her eyes for a moment, then nodded. 'All right. Kitig, how many mammal species are there on your alternative world?'

'On Paratractis? I'm not sure. Several thousand at least.'

'Jacob's virus, or more accurately, the prions that go with it, will kill all mammal species all mammal species or at least most of them. So something before this time wiped out humanity if it was happening afterwards, there wouldn't be any mammal species on Paratractis. And I have to find out what it is, while the vortex is still stable enough to allow any sort of time travel.' or at least most of them. So something before this time wiped out humanity if it was happening afterwards, there wouldn't be any mammal species on Paratractis. And I have to find out what it is, while the vortex is still stable enough to allow any sort of time travel.'

Jo shut her eyes for a moment, tried to think it through. It was as hard as ever to make sense of the Doctor's babble.

'Do you mean Jacob can't possibly succeed?' she asked at last.

'No. It's a neither-or-either loop. If I fail to stop whatever's happening before before this time, then it won't matter what you do. this time, then it won't matter what you do.

If I succeed, then Jacob could still be responsible for destroying the human race in this this time. Somebody was taking no chances.' time. Somebody was taking no chances.'

'So?'

'You've got to find Sam. And between you, you've got to stop Jacob.'

'And you're going to go off and do something else?'

The Doctor nodded. 'That's it!'

Jo shook her head. 'Nothing changes, does it?'

'I'm sorry, Jo. There isn't any choice.'

'Well, you said from the start that you needed my help. That's why I got mixed up in this.'

The Doctor frowned. 'From the start? Did I? When did I do that? I mean, perhaps I'm going to do that I can never be sure.'

Jo shook her head again. 'Never mind.' She walked past him, towards the bloody mass on the ground under the shade of the alien tree. 'Now if you'll excuse me, I'm going to say goodbye to my friend.'

Sam and Jacob found the habiline not far up the slope, collapsed in the shade of a rock.

Jacob lifted the habiline's arm and put the needle against it. 'Here, can you hold this thing's fur out of the way?' he asked.

'It needs to be shaved off, but there's no time.'

Sam stepped forward.

And the habiline moved.

Jacob pitched forward, yelling. The habilines dark hand clamped around his arm. Its other hand made a wavering arc towards his throat.

Jacob lowered the knife, but missed as the habiline rolled aside. The animal carried on rolling across the hard stone, almost knocking Sam off her feet. She jumped up, saw Jacob raising the knife to throw it but it was too late: the habiline had vanished into the rocks.

A movement.

Something flew through the air, aimed at Jacob's head.

'Look out!' yelled Sam, but Jacob was already moving. The stone clattered past him. Sam saw black flint, a sharp edge.

Then there were habilines everywhere, screaming, waving their arms. Stones flew, clattering on the rocks. Sam felt one punch her side.

An ambush, she thought. Definitely almost human.

'Run!' she bawled at Jacob, but he was waving the syringe around, looking for a habiline to stab.

He looked over his shoulder, shouted something, pointing behind Sam. A rock hit her leg with bruising force. Another landed just ahead of her, clattered back on to her hand. She grabbed it, hurled it at the nearest habiline.

He caught it, advanced on her with the stone stone axe in his hand, sharp end forward.

'Wait!' If these things had any kind of language, there was a chance that the TARDIS translation system would work with them. 'Wait! I can help you!'

The habiline paused, and an expression appeared on its face that Sam could only characterise as puzzled.

Then something hit Sam on the head, and she felt her legs give way beneath her.

Kitig watched as the Doctor finished shovelling earth over the bodies of the two women. He was still amazed by the room he was in: or rather, the space space.

It was hard to believe that they were still inside the Doctor's time machine. There was a hillside. A distant horizon. And there were millions upon millions upon millions of butterflies, making clouds in the blue, sunlit sky. Some of them clung to the Doctor. Others drank greedily from the damp fresh earth of the graves.