"Use that," he said. "And hold it the other way up or its ears will come off in your hands when you hit it."

Grabbing the hare by its back legs, Tris hung the animal upside down and thumped it hard with a stone on the back of its head without giving herself time to think. Shitting black raisins at her feet, the animal turned from something living to meat.

"You do the rest," Tris said.

Fifteen billion people watched her toss the dead hare at Luca's feet, although Tris didn't know this. Which was just as well, because the first thing she did after stalking from the camp and dry-vomiting away her disgust, was drop her silk trousers and raise the hem of her padded jacket, letting rivers of steam melt frosted blades of grass.

"Moron," she said.

And all the while, buzzards circled overhead and a lizard clung to rock, either dead or too catatonic with cold to move. There was no single camera watching Tris and Luca. Indeed, the concept "camera" meant nothing to Tris. If she'd stopped to wonder how feeds were fed she'd have decided by magic.

The truth was far stranger. Every living thing on Rapture watched everything else, from the cat that slunk across the yellow roof of the Emperor's pavilion to the single butterfly delivering a message as it touched his wrist. And the Library drew together these threads and, from them, created a seamless feed that was life in the Forbidden City.

Ripping a leg from the roasted body of the hare, Luca held it out as an offering. "Try it," he suggested.

They ate in silence.

It sleeted that night and again the next morning. What had started as sleet became hail, driven on a chill wind that roared down a valley into their faces. They had to set their next bivouac quickly and break it down just as fast, Luca converting their crude tent back into his cloak with a sleight of hand that Tris somehow always missed.

"You sure this is the right way?"

"No," said Luca, "I'm not."

"We should have brought a map."

Luca stared at the hail and sleet breaking up the world around them. "No point," he said. "Coordinates have zero meaning at this level." It was the last thing he said that day.

And Tris was ready to believe he'd forgotten her existence, except that once she slipped while stepping from rock to rock and Luca grabbed her so fast she barely saw his hands move. She slept in his arms that night as snow piled up against one side of the bivouac, although there was nothing sexual in his stroking of her hair and both retained their clothes.

"No," Luca had told her, when Tris first knelt to scrape snow from the hillside, making space for their bivouac. "Don't dig."

"Why not?"

"Sleeping on snow is warmer," he told her. "Here..."

Tris caught his knife.

"Stab the ground."

Shock echoed up Tris's arm and only the fact Luca's knife had a crossbar stopped Tris slicing her hand on the blade.

"That little sword of yours could break stabbing this stuff," said Luca. "It's permafrost. You need to know these things."

He read the question in her face.

"Because," Luca said, "you're meant to be doing this on your own."

The dreams were worse that night. So terrible that when she woke Tris would not allow herself to remember a thing. All she could feel was their numbness, as if the permafrost over which she slept had entered her soul. Having eaten the last scraps of roast hare without tasting, Tris reached for Luca and pulled him close.

"I'm not sure this is wise," said Luca, opening one eye.

Tris reached down with her hand. "You know what?" she said. "I'm not sure I care."

Afterwards, Luca scrambled out from under the cloak and disappeared behind a low strand of bushes. "Now you," he said on his return.

"It'll hold."

"No." The Baron shook his head. "It won't... From here on when we climb we're tied together. You want a piss, I'm this far away." He held his hands so, indicating distance.

In fact the gap between Luca and Tris as they climbed the first snow bank was greater than Luca had said it would be, if not by much. And Tris wore the stolen blade across her back, because Luca had insisted she take a long stick of thorn in each hand, so that if Tris missed her step she could jab her sticks into the snow and avoid sliding back the way she came. He also made her walk first, on the grounds that if she did slip he might be able to catch her.

The dreams haunted her again that night and followed her into the day. All Tris got were glimpses from the side of one eye. Patches of snow that kept pace, stalking the edge of her vision where endless flakes of falling snow lost themselves in a perpetual half glow that ice fields seemed to bring with them.

Once she saw something stranger.

Amber eyes like Luca's, but staring from the face of a huge cat. She told Luca about this and in return he told her about snow blindness, hypothermia, oxygen starvation and their collective responsibility for her hallucinations. He left out the pain, Tris noticed, and after a few minutes she zoned him out and concentrated on climbing the icy slope in front of her.

Every now and then, Tris would thrust one hand inside the front of her padded jacket and nestle it under her armpit in an attempt to thaw out her fingers and once, when Luca was looking at something else, she thrust both hands between her legs. The pain of her fingers unfreezing hurt so much that tears crystallized on her cheeks like pearls.

Around midday they stopped climbing, the snow underfoot levelled out and then began, very gently, to dip in the opposite direction.

"That was it?" said Tris as she unknotted Luca's rope and dropped her end in the snow. "That was your cliff?"

Luca frowned. "Tristesse," he said heavily, "we've barely started."

He wouldn't look at her for the rest of that afternoon and, come evening, he just scooped out a shallow dip in a snowdrift, did whatever he did to his cloak and buried the edges of the newly created bivouac beneath the snow to keep them secure. He made no attempt to start a fire, nor did he invite Tris inside when finally he crawled under the cloth.

After a few minutes, Tris clambered inside anyway.

They slept like husband and wife, back to back, not touching. It was an old, sour joke from her grandmother. One she'd failed to understand until that night, the night the snow tigers came.

When the first animal padded silently out of the darkness, Tris was restless and already awake. The tiger came in a gap between falls of snow. A handful of white shadow and smoke-grey stripes, paws the size of plates carrying it over a skim of frozen crust, its tail brushing the snow as it loped out of the darkness and halted outside Luca's make-shift tent.

The others came in the seconds which followed.

It was their breathing Tris heard first. "Me?" she asked, in case there was some mistake. And the biggest of the tigers nodded, fat strands of spittle drooling onto pale snow.

"Malika," it said when Tris stayed where she was.

"I'm Tris," said Tris. She wasn't too sure they'd got that bit.

"Malika," repeated the tiger.

She went to it anyway, crawling from beneath Luca's bivouac and walking barefoot over the snow crust, leaving lonely footprints behind her. All three were beautiful, elegant beyond anything life had let Tris imagine. Their eyes amber and their claws tallow, like ancient ivory.

"You're beautiful," she said.

The biggest tiger's casual nod seemed to suggest that this was obvious.

"Can I feel?" Reaching out Tris tangled her cold fingers into warm fur. And as soon as her hand gripped the tiger's mane, the beast began to move, slowly but decisively.

"She's going," said a voice.

"Not much we can do about it now." That voice was different. Come to that, so was the voice before. Rougher, speaking words Tris barely understood.