She let go of the rollbar, touched Julie's arm. 'Sorry.'

Julie nodded abstractedly, the incident already forgotten. She was peering out at the sloping walls of the gorge. 'I think it's about here. These strata are about two and a half million years old, and the fragment was oh!'

The car gave a particularly violent jolt and stopped suddenly.

Rowenna looked around, saw dust settling, the cracked mud and litter of broken rock that made up the seasonal riverbed, and the familiar eroded bluffs that her experience and her increased heartbeat told her were fossil country.

Her eyes scanned the nearest of them, pain forgotten, Julie forgotten, everything forgotten but the possibility of a find.

And then she saw it, and realised why Julie had stopped the car so suddenly.

Less than twenty yards away, embedded into the sloping bank of the riverbed at just above eye level, was a knob of rock that wasn't all rock. The curve of a cranium, the staring socket of an eye. That was all: the rest was still hidden in the sediment, or perhaps eroded away.

But it was enough. Even from this distance, and through the windscreen of the car, Rowenna knew that she was looking at the skull of a hominid very different from any that would normally be present in strata this old. In fact it almost looked like...

'Wow,' said Julie. 'That wasn't there this morning.'

Rowenna glanced at her, frowning, annoyed to have her train of thought interrupted. 'You mean you missed it.'

Julie shook her head. 'Hell, Rowenna, it wasn't there. I walked down this section. I saw that fragment. I'm damn sure I'd've seen a complete skull.' A pause. 'Least, I think think I would. I'm going to take a look. Want me to load you up into the chair?' I would. I'm going to take a look. Want me to load you up into the chair?'

Rowenna looked at the rough ground between the Land Rover and the fossil. 'I don't know whether the chair's motor will take it.'

Julie grinned. 'It will if I push. Hell, all that food energy has to be good for something!' She got out of the car, walked around to the back and opened the door.

A stone bounced off the bonnet, and at the same time Rowenna saw a movement on the rock bluff above the fossil skull.

A man. A man making his way down the bluff. She could see the blue uniform, the shoulder pips.

She heard an intake of breath from Julie, heard her footsteps as she marched round the car to meet the soldier.

'Whoever the hell you are, keep away from that skull!'

Rowenna grinned. Good old Julie, diplomatic as ever. She wound down the window as the soldier trotted across to the car. He was white, tall, with a thin fuzz of blond hair.

Definitely not local: the blue uniform looked like it was a United Nations kit.

'Ignore my friend,' she said. 'Can we help you?'

'I'm sorry, I'm going to have to ask you to leave the area.' The man sounded worried.

Rowenna frowned, but Julie spoke again before she could frame a reply. The big woman had marched around the car and was facing up to the soldier.

'Can I see your authorisation for that?' she said. 'We're here on legitimate '

'I'm sorry, you've got to leave. Now.'

'And I want to see your authorisation!'

Rowenna reached out through the window and put a hand on Julie's arm. 'Hold it '

But the man had produced an ID. Julie studied it. 'Intelligence Taskforce?'

The man nodded. 'This entire area is a restricted zone at present.'

'Why?'

'I'm afraid I can't say why.' The ghost of a smile. 'That's why it's restricted. Now, can I ask you to leave, please?'

Rowenna decided to try again. Whatever military nonsense was going on here, the fossil was too delicate, too rare to be exposed to it. 'We're palaeontologists,' she said. 'We only want to have a look at the fossil that one, there. We aren't interested in any military stuff. You can stay and watch us if you want.'

'Sorry, ma'am, I have my orders. You have to leave now.'

'Can we check that with your senior officer?' asked Julie.

That ghost-smile again. 'I am in charge, here, now.'

'I want the name of your senior officer!'

'I'm not at liberty to tell you that.'

Again, Rowenna touched Julie's arm. There was a look in the officer's eyes that she didn't like, a coldness that reminded her turning, the gun in his hand, the gun 'Let's go,' she said.

Julie glanced at her. 'Are you crazy? That skull's got to be '

'Please.' Rowenna could feel the panic growing, like an animal in her chest struggling to get out.

Julie glowered at the soldier, then walked slowly around to the driver's door. 'We'll be back, just as soon as your military nonsense is over,' she said. She started the engine, reversed the Land Rover. Rowenna caught a last glimpse of the skull, staring at her one-eyed, like a warning.

Then Julie turned the Land Rover around.

'I didn't like that man,' said Rowenna quietly.

'Neither did I,' said Julie. 'I'm going to make an official complaint.'

'I mean, I was afraid of him.'

Julie glanced across at her. 'Yeah, I know. Don't worry, I got his name. Captain Jacob Hynes.'

Jacob watched the two women driving away.

Damn, he thought. Damn it all to hell. We don't need fossil-hunters. I knew I should have closed off the gorge this morning.

He lifted the satphone to his lips, muttered the voice ID that opened the local uplink. After a few seconds, the UNIT logo and a security code appeared on the machine's small screen, together with the words BUSY LEAVE MESSAGE.

'Brigadier General? Hynes here, at Kilgai. I need the hack-up platoon down here. We have several new anomalies, and there are civilians around, palaeontologists. They might blow the whole thing open. I need an ETA ' He paused, deliberately theatrical. 'I need those people, sir.'

He cut the connection, put the radio away in his pocket.

Then he waited for a while, looking at the fossil skull in the bluff, until he was sure that the Land Rover had gone, that the fossil-hunters weren't going to try to sneak back. Finally he scrambled up the bluff to his makeshift camp in a narrow cave at the bottom of the cliffs.

Inside, behind the UNIT standard-issue portable stove, the folded canvas awning, the rolled-up sleeping bag, the time tree glittered, its branches snaking with light, the seed clusters ripe.

Pick one and you go back a thousand years. Or ten thousand years. Pick a hundred and

Jacob smiled to himself.

He reached under the sleeping bag, pulled out the other other radio. It was a sleek, strange, organic-looking machine with a surface like polished agate. It moulded to his hand. There were no buttons, no controls, no screen. Once he was touching the machine, he needed only to speak. radio. It was a sleek, strange, organic-looking machine with a surface like polished agate. It moulded to his hand. There were no buttons, no controls, no screen. Once he was touching the machine, he needed only to speak.

'Gavril?'

The response was immediate, the familiar synthesised voice. 'I am here.'

'I think ' This time the pause was not deliberate. 'I'm almost ready to go. I've sent for the UNIT people to seal off the gorge.'

'And the tree?'

'It's OK,' he said. 'Ready to go.'

'Good. You have chosen the correct moral path, Jacob Hynes. You will be honoured for all time by my people.'

Jacob nodded, but he wasn't interested in honour, nor in Gavril's people.

Alpha and Omega. A dream come true. A dream come true.

CHAPTER 4.

The Tractite city was even more beautiful from directly above: the buildings were like misted glass eggs, illuminated from within, and there were amorphous coloured forms inside the walls, like embryos with green and amber and blood-coloured flesh. The paths between the buildings glittered, showing tiny fragments of colours: pale blue, lavender, silver, and gold.

Four Tractites drifted below, standing in a bright-green field in what must be artificial light, throwing a small, glittering object between them; every time one of them caught it, it chimed. Even the mist seemed beautified by the city, transformed into a slightly coloured haze. Gauzy clouds formed around the lights as they approached, dissipated as they passed by.

Sam heard the Doctor's voice, talking to the Tractites, the faint snorts and whinnies of their responses.

She focused on the conversation, realised that the Doctor was talking to Narunil about spaceships. The Tractite was sounding puzzled: every type of ship the Doctor mentioned seemed to be something she hadn't heard of. Sam was sure that the Doctor was nosing around for information, and equally sure that what he was trying to find out didn't have anything directly to do with hyperspace or warp drives.

Well, she would work it out. And one thing was sure: she wasn't going to ask, not unless she absolutely had to.

Cold, damp air hit Sam in the face as the force field around the skimmer was powered down. They had landed. The city looked less ethereal, now that she was on a level with it. Bubbles had become domes, some of them several storeys high.

Paths had become streets, lamps had become brilliant streetlights. Tractites were clopping about in the white glare of the lights, their breath pluming in the air, and there was a bustle of carts being loaded and unloaded, of coloured fabric rolls and wicker baskets and food bubbling in metal pots. Despite the cold, the air smelled like a summer garden, sweet grass and soil and flowers.

Kitig had parked the skimmer in the middle of a large circular intersection. Several paths looped around them, and there were even flyovers though they could hardly be further from the concrete monstrosities of human make. They looked as if someone had thrown an arc of water through the air and frozen it to pastel-coloured, glinting ice. Connections flowed from the bridges to the ground, and into adjacent buildings.

As they followed a path towards one of the bridges, Sam noticed that there were flowers growing from narrow beds of soil around the supporting pillars. They looked as if they were cast from wax in the pale light, but when she knelt to touch them they were soft, real flowers, and unmistakably terrestrial: daffodils. Looking around, she saw winter jasmine, wall-flowers, crocuses. But there were subtle oddities about them: the crocuses came in bright red, as well as the usual purple and yellow. And almost all of the daffodils were pure white.

Sam stopped, took a closer look. Surely they were were daffodils? Everything else was right: the shape of the flowers and of the leaves, the length and colour of the stalks. daffodils? Everything else was right: the shape of the flowers and of the leaves, the length and colour of the stalks.

'I see you've noticed too.'

Sam jumped. The Doctor was crouching beside her, his breath frosting the air. Sam nearly said, 'Noticed what?' but stopped herself just in time.

The Doctor gently touched the petals of the flowers. 'Almost the same. And everything else is so different. It's as if there's some fundamental instability ' He stood up. 'Never mind. Even simplicity itself is never as simple as it seems.' He started after Narunil and Kitig on a curving path that led under the bridge and into a small, bright street lined with beds of the white daffodils.

Sam followed him. Two huge Tractites cantered past her on the path and began ascending the gentle incline of the bridge. Sam felt their eyes on her, but tried to ignore the feeling. It was too much to expect that these people wouldn't be curious. There didn't seem to be many other aliens around.

Sam realised what she'd been thinking and stopped suddenly, staring at the flower-lined street in front of her.

Aliens.

And this is Earth. These are daffodils, and I'm an alien. Sam frowned, then remembered something about the Doctor's conversation with Narunil on the skimmer. He'd mentioned the Bror coast on Tractis. True, he'd pretended to be confused and absent-minded, but Sam was pretty sure that he'd been faking that. So presumably the Bror coast was a real place, and very well-known and Narunil hadn't heard of it. So either she'd been lying about having visited Tractis which seemed pointless or the Tractis she knew wasn't the same as the Tractis the Doctor knew.

Just like this Earth isn't the same as the Earth I come from. Different flowers. A different intelligent species.

She tried to remember what the Doctor had told her about the nature of time, and quickly realised that it wasn't much.

But she'd always had the impression that time was relatively constant that the TARDIS was a sort of shuttle craft moving from one century to another or one planet to another, using pretty much the same mechanism for both sorts of transition.

Yes, they could make small changes, like saving a child's life. But what if they could make larger changes? Changing the outcome of a major war, for instance.

Yes! That was it! She felt a weight lifting from her shoulders. Earth hadn't been invaded at all!

'Doctor! I've got it!' she shouted, racing along the street after him.

The Doctor turned from where he was walking between the two carthorse-sized Tractites.

'We're in an alternative universe!'

The Doctor frowned and shook his head, glanced at Narunil and muttered something. Then he walked back to Sam and grabbed her by the arms.

'Sam, Sam, Sam, Sam Sam,' he said, speaking in a low voice. 'Do please please be careful what you say. Yes, we are in an "alternative universe", but the trouble is, it's inherently unstable for a reason I haven't yet understood. And if it collapses, it won't just disappear, it's far too big for that it will create a rift in the vortex so huge that the whole fabric of reality will collapse.' be careful what you say. Yes, we are in an "alternative universe", but the trouble is, it's inherently unstable for a reason I haven't yet understood. And if it collapses, it won't just disappear, it's far too big for that it will create a rift in the vortex so huge that the whole fabric of reality will collapse.'

Sam opened her mouth to object, but the Doctor wasn't stopping.

'And before you ask, no, we can't just go away and pretend it isn't happening. I have to fix this, and the only way of doing that is if I can somehow find out when things changed, and how, and change them back but given how far back that's likely to be, and how much is likely to have changed, there may well be nothing I can do, and even if we do try anything, our kind hosts will probably try to defend their own existence, which means stopping us from doing it, perhaps by killing us. So it really would be a good idea if you didn't tell them about it in advance. Because if I can't fix it then neither you nor I will ever meet another living human being again, and we, and the Tractites, and everything else, will probably cease to exist by about next Tuesday. Now do you understand how serious this situation is?'