'Marked?'

Kitig took the book from Sam, brushed the mould away from part of the cover, then pointed at a heart-shaped mark in the top left corner. He showed it to Sam. 'That means the book's finished with. We usually just put it back on the soil it's quicker.'

Sam grinned at the alien. She looked around, and saw that there were large numbers of books scattered around on the soil under the plants, and that most of the things she'd put down as dead leaves were in fact stray pages.

'Recycling!' she said. The more she learned about these people, the more she liked them. But then she thought for a moment and frowned. 'What happens if you want to keep something, though? I mean, everything must go mouldy pretty soon in here. Do you reprint it?'

Kitig gestured at the librarian. 'They make a new copy, changing anything that needs to be changed. We do use printing, sometimes, if a large number of copies are needed. There's a press over by the high table.' He gestured beyond the fountain, to a tall stand of trees. Sam could see glimpses of bright stonework between the trunks.

But she was still confused. The Tractites seemed too advanced for this. Where were their computer systems?

'Haven't you got a permanent store of information?' she asked.

'How can information be permanent?' Kitig seemed genuinely puzzled. He shut his eyes and made a slight tossing motion with his head, very horselike, then said, 'Information is what people need in order to know about the world. The world is always changing, therefore the information has to change as well, or it's no longer information. The books are copied for as long as they're needed. When they're not needed any longer when they're no use to anyone what's the point in keeping them?'

Sam thought about it. 'OK, not permanent. Long-term. Like physics and stuff.'

Kitig's eyes clouded for a moment, and his tongue tasted the air. 'Honeysuckle, I think,' he said. 'I can taste some here.

Did you want to look at them?'

'Judging by their antiquity,' the Doctor broke in, 'I should think that the Tractites have forgotten more about physics than the human race ever knew.' He was lying on the floor, sniffing at a small, diamond-shaped book with pages stained purple and pale yellow. Sam couldn't see any writing, as such, and eventually decided that the colour patterns must constitute words.

Or maybe it was the smell.

She looked around again, observed the librarian more carefully.

Yes. He was tasting the book, his tongue flicking out to touch it, then moving to the new book and presumably copying the information. The process was quite rapid, perhaps ten pages a minute. Sam wondered how much information was on a single page.

'Colours set the framework, taste is used to convey specific events,' commented the Doctor after a while. 'Apparently they've been doing it for over two million years.'

'Must be a pretty good system then,' said Sam, as usual refusing to be fazed.

But two million years two million years? How long had the Tractites been here? It was beginning to look as if, in this universe, the human race had simply never existed.

She glanced at the Doctor, hoping he would say something more about the Tractites' apparent antiquity, but he was still sniffing the book and talking about language. 'Well, it's better than a limited set of shapes, a bizarre set of combinatory rules and a syntax where you can't do more than two clauses before Hello, this is interesting. Who's the Uncreator?'

'The mythical entity who brings about the End of Time,' said Kitig, before Sam had even managed to pick up the broken pieces of the Doctor's own syntax and work out that the final question had been addressed to the alien.

'Ah,' said the Doctor. 'Hmm.' He sprang up and beamed at Sam. 'Time for that tea that Narunil promised us, don't you think?'

Not a very adroit change of subject, thought Sam. Obviously there's something in that book that the Doctor doesn't want Kitig to know he's especially interested in. Something that gives a clue to how we're going to have to change the past and Destroy this universe.

Sam suddenly felt cold in the warm, humid air. Her interest in the alien library and its books had made her forget all about the fact that the Doctor was going to destroy this entire world if he got the chance.

It's like knowing you're going to die, she thought. You don't think about it most of the time, then suddenly you remember, and your heart starts thumping, and you feel sick. Except that death is something that happens to everyone, and I can't do anything about it, so I can just tell myself not to think about it. But this is only happening to me, and I have to think about it.

We're going to kill the Tractites. All of them.

The Doctor was looking at her curiously. Sam realised that her emotion must be showing on her face.

'Just homesick,' she said quickly.

'Homesick? You?' He took her arms, looked into her eyes, his gaze earnest and reassuring. 'Oh, Sam, Sam, Sam. It'll be all right. I'll find a way.'

Sam nodded, bit her lip, broke away.

Kitig gave her a sympathetic glance. 'I'm sure you'll be able to go home,' he said. 'We do have spaceships, though not at Afarnis; I'm sure our people can lend you passage on one, if yours can't be repaired.'

Kitig, of course, thought 'home' was another planet, because that was what the Doctor had let him assume.

More lies. And I'm helping him, just by keeping quiet.

Kitig touched her arm with his big, three-fingered hand. It almost broke her heart.

She glanced at the book that the Doctor had been reading, now discarded in the pile on top of the table. She wondered what was in there that was so important.

'And you're sure there's never been a species that looked like us on this planet?' the Doctor was saying. 'Not even in the fossil record?'

'There were some bipeds,' Kitig replied thoughtfully. 'On one of the equatorial continents, I think. They'd have been about your size. But as far as I know, they're extinct. And they weren't intelligent.'

'Obviously I must have got it wrong again,' muttered the Doctor.

'Not necessarily,' said Kitig politely. 'The fossil record is patchy; there may well have been a small alien colony here.

But it would have to have been a long time ago.'

'When did you come here, then? You're not natives, I take it.'

Sam could sense the sharpness behind the apparently casual question. She opened her mouth to interrupt, then thought better of it, and looked away into the dripping foliage.

'That's a bit of a puzzle,' Kitig said. 'We have records going back thousands of generations, you understand. There are even Tractite fossils here on Paratractis. But the strange thing is, Tractites definitely evolved on Tractis, not here. There are genetic differences between us and most other Paratractian species that can't be explained any other way. There are a lot of theories about it our civilisation on Tractis may have been overrun, after seeding the Earth colony, and later rebuilt itself.

There's even a theory involving time travel '

Sam glanced at the Doctor again, but his face gave nothing away.

' though that's always seemed paradoxical to me. "I'm my own grandpa." You know.'

'Yes, we have that saying on our planet as well,' said the Doctor. His voice was distant, as if he were thinking of something else. Sam watched his face, looking for clues.

Suddenly he seemed to come to a decision. He set off along the path, obviously expecting Sam and Kitig to follow. Kitig did, but Sam didn't. She walked a little way, waited until the Doctor was out of sight, then dropped back and scooped the mysterious book up from the table where the Doctor had left it. She opened it, looked at the meaningless patterns on the pages for a moment, then cautiously put it to her mouth and licked the soft material. It wasn't papery: more spongelike in texture, and it had a faintly metallic taste.

No something more than metal.

Meat.

Cooked meat no, cooked flesh. Burnt Burnt flesh. Corpses rotting, orange worms burrowing in it, bodies piled high against a flesh. Corpses rotting, orange worms burrowing in it, bodies piled high against a wall wall Sam recoiled, feeling sick. She could see why the Doctor hadn't wanted her to know about this. It wasn't going to be pleasant. The worst aspect was that she seemed to be actually experiencing what she tasted, rather than simply reading words.

But she had to find out.

She screwed up her courage, took another taste A city, drowned in a cold blue-white glare. All the roofs and streets visible. Tiny figures staring upward, Tractites, caught in the shadowless illumination like insects in amber caught in the shadowless illumination like insects in amber And then the light. Light burning her eyes, burning her flesh, her bones, searing, destroying Sam turned a page. She was feeling dizzy, and her throat was hurting, but she had to know more. She took another taste A black-carapaced alien walking towards me, the band of plastic over its eyes glowing an uneven red. Fire leaps towards me and the searing light, the pain towards me and the searing light, the pain The humans are coming! The humans! They're going to kill us all!

'Sam!'

Sam felt the blow on her face, and for a moment she almost tried to kick out with forelegs she didn't have. Then she became aware of the library around her, of the Doctor's anxious face, of Kitig staring at her.

'I I was a Tractite,' she said. 'Several Tractites.'

The Doctor was shaking his head, putting a finger to his lips.

No, thought Sam. You need to know this.

Then it occurred to her that the Doctor already did know. It was the Tractites she was supposed to be keeping the secret from.

She was getting tired of this.

'I don't know what was happening,' she said. 'There was just a lot of pain '

The Doctor nodded vigorously, as if encouraging a toddler to walk. 'It's all right,' he said. 'You don't need to worry about it now.' He turned to Kitig. 'She'll be fine, I think,' he said airily. 'But some of your fiction is a bit gruesome, isn't it?' He tossed the book down on the ground and trotted off along the path.

Sam hesitated, then followed him. They passed the high table in its tamarind grove. The printing press, a big, wooden-framed machine with metal springs and coils and cogs and levers, seemed to grow out of one of the trees. Glass retorts hung suspended from the framework, stained with various coloured fluids.

For putting smells on to paper. Of course.

Another machine, smaller, was set in a niche, like a classical statue. It looked a bit like a large vice: perhaps it was for binding the books. The Doctor paused to look at it; Kitig went ahead.

Sam caught the Doctor's arm. 'Was it fiction?' she asked.

The Doctor turned and looked at her glumly. 'No. That's the problem.'

Then he marched on after Kitig.

Sam stared after him for a moment.

If it's not fiction then there must have been humans in the past of this universe, or maybe that record is from our universe, in which case In which case she hadn't the faintest idea what was happening. Thinking about it made her head spin.

There was an exit behind the high table; a librarian sat there, copying a book. Sam noticed that her fur was patchy, dry, like that of an old cat or dog. And she looked looked old, bones protruding from wasted flesh under her white robe. old, bones protruding from wasted flesh under her white robe.

She looked up suddenly and glared at Sam. 'The scents have changed,' she said. 'It's all your fault.'

Kitig laughed, the sound booming around the chamber like thunder. 'Oh, Partil, the scents are always changing for you!'

The librarian shifted her glare to Kitig. 'That's just the problem,' she said. 'They didn't always change.'

'Don't take any notice of her,' muttered Kitig when they were outside. 'Her sense of taste is gone has been for years.

They only keep her on because her daughter's on the library council.'

Sam bit her lip. Even in their weaknesses, these people were more human than humans.

She thought about the book again. If it wasn't fiction then humans had already destroyed the Tractites once or tried to.

But if this was 2108 that would have to have been in the twentieth century too early in human history for interstellar travel, let alone conquering alien planets.

Unless the humans came from somewhere else. Unless the Tractites came from somewhere else.

And there were two universes to consider.

Sam shook her head. It was no use: she wasn't going to be able to make sense of this without drawing a diagram. Probably not even then.

She followed the others outside and took a deep breath of the damp, marshy air. She looked at the beautiful city that the Tractites had built here in the middle of a wasteland. The Tractites were good people better, probably, than humans.

However they had arrived here, they showed every sign of being good managers of Earth and its resources. Did she and the Doctor have the right to destroy them? What right did either of them have to start unpicking the threads of the new history that had formed around them?

She looked at the Doctor. He was still talking to the aliens, but, watching closely, she thought she could see it now: a crafty expression on his face, underneath that charming smile.

He was going to do it. If he could, he was going to change history. Of course he was. And she was going to help him.

She was going to have to; she didn't have any choice.

According to the Doctor.

Perhaps if I don't think about it, she thought, it will all go away. The Doctor will have put it all right and I won't have to do anything.

But she knew there wasn't much chance of that.

CHAPTER 8.

The fax machine started just as the helicopter was coming in to land. Jo stared at the curl of paper, watched as the words TOP SECRET rose slowly over the tiny plastic horizon of the portable machine, followed by a string of security codes.

She glanced out through the small porthole of the military helicopter, saw low concrete buildings, a woman in shirtsleeves waving the aircraft down in the middle of a storm of dust. She looked over the roofs of the buildings for the hills that surrounded the famous gorge, but saw only a featureless red plain dotted with thorn trees.

The machine in her lap beeped. Jo looked down. The body of the fax was in John Benton's neat handwriting: ' Jo Computer record of Captain Jacob Hynes back to June '81 but no record on Jo Computer record of Captain Jacob Hynes back to June '81 but no record on microfilm. You were right. It sounds dodgy to me. I'll take it up with Brigadier Bambera microfilm. You were right. It sounds dodgy to me. I'll take it up with Brigadier Bambera ASAP. Take care John' ASAP. Take care John'

Jo shook her head slowly. Once, you'd have come out after me yourself, she thought. But that was a different world. A galaxy long, long ago and far, far away.

The helicopter was down now, the dust settling. The door clattered open and the shirtsleeved woman peered inside.