She caught up with Durham just as he reached an ornate curved staircase; they were upstairs in what seemed to be a large two-story house. He clapped her on the shoulder. "Thank you. Try downstairs, I'll keep going up here."

Maria wished she'd disabled all her human metabolic constraints -- but she was too agitated now to try to work out how to make the changes, too awash with adrenaline to do anything but run down corridors bellowing, "Is there anyone home?"

At the end of one passage, she burst through a door and found herself out in the garden.

She looked about in despair. The grounds were enormous -- and apparently deserted. She stood catching her breath, listening for signs of life. She could hear birdsong in the distance, nothing else.

Then she spotted a white shape in the grass, near a flowerbed full of tulips.

She yelled, "Down here!" and hurried toward it.

It was a young man, stark naked, stretched out on the lawn with his head cradled in his hands. She heard breaking glass behind her, and then a heavy thud on the ground; she turned to see Durham pick himself up and limp toward her.

She knelt by the stranger and tried to wake him, slapping his cheeks. Durham arrived, ashen, clearly shorn of his artificial tranquility. He said, "I think I've sprained an ankle. I could have broken my neck. Don't take any risks -- something strange is going on with our physiology; I can't override the old-world defaults."

Maria seized the man by the shoulders and shook him hard, to no effect. "This is hopeless!"

Durham pulled her away. "I'll wake him. You go back."

Maria tried to summon up a mind's-eye control panel to spirit her away. Nothing happened. "I can't connect with my exoself. I can't get through."

"Use the doorway, then. Run!"

She hesitated -- but she had no intention of following Durham into martyrdom. She turned and sprinted back into the house. She took the stairs two at a time, trying to keep her mind blank, then raced down the corridor. The doorway into the evacuation control room was still there -- or at least, still visible. As she ran toward it, she could see herself colliding with an invisible barrier -- but when she reached the frame, she passed straight through.

The clock on the interface window showed twenty seconds to launch.

When she'd insisted on hanging around, Durham had made her set up a program which would pack her into the new Garden-of-Eden in an instant; the icon for it -- a three-dimensional Alice stepping into a flat storybook illustration -- was clearly on display in a corner of the window.

She reached for it, then glanced toward the doorway into Riemann's world.

The corridor was moving, slowly retreating. Slipping away, like the buildings of the City.

She cried out, "Durham! You idiot! It's going to implode!" Her hand shook; her fingers brushed the Alice icon, lightly, without the force needed to signal consent.

Five seconds to launch.

She could clone herself. Send one version off with the rest of Elysium, send one version in to warn him.

But she didn't know how. There wasn't time to learn how.

Two seconds. One.

She bunched her fist beside the icon, and wailed. The map of the giant cube flickered blue-white: the new lattice had begun to grow, the outermost processors were reproducing. It was still part of Elysium -- a new grid being simulated by the processors of the old one -- but she knew the watchdog software wouldn't give her a second chance. It wouldn't let her halt the launch and start again.

She looked back through the doorway. The corridor was still sliding smoothly away, a few centimeters a second. How much further could it go, before the doorway hit a wall, stranding Durham completely?

Swearing, she stepped toward it, and reached through with one hand. The invisible boundary between the environments still let her pass. She crouched at the edge, and reached down to touch the floor; her palm made contact with the carpet as it slipped past.

Shaking with fear, she stood up and crossed the threshold. She stopped to look behind the doorway; the corridor came to a dead end, twelve or fifteen meters away in the direction the doorway was headed. She had four or five minutes, at most.

Durham was still in the garden, still trying to rouse the man. He looked up at her angrily. "What are you doing here?"

She caught her breath. "I missed the launch. And this whole thing's . . . separating. Like the City. You have to get out."

Durham turned back to the stranger. "He looks like a rejuvenated Thomas Riemann, but he could be a descendant. One of hundreds. One of millions, for all we know."

"Millions, where? It looks like he's alone here -- and there's no sign of other environments. You only discovered one communications port, didn't you?"

"We don't know what that means. The only way to be sure he's alone is to wake him and ask him. And I can't wake him."

"What if we just . . . carried him out of here? I know: there's no reason why doing that should move his model to safer territory -- but if our models have been affected by this place, forced to obey human physiology . . . then all the logic behind that has already been undermined."

"What if there are others? I can't abandon them!"

"There's no time! What can you do for them, trapped in here? If this world is destroyed, nothing. If it survives somehow . . . it will still survive without you."

Durham looked sickened, but he nodded reluctantly.

She said, "Get moving. You're crippled -- I'll carry Sleeping Beauty."

She bent down and tried to lift Riemann -- Thomas or otherwise -- onto her shoulders. It looked easy when firefighters did it. Durham, who'd stopped to watch, came back and helped her. Once she was standing, walking wasn't too hard. For the first few meters.

Durham hobbled alongside her. At first, she abused him, trying insincerely to persuade him to go ahead. Then she gave up and surrendered to the absurdity of their plight. Hushed and breathless, she said, "I never thought I'd witness . . . the disintegration of a universe . . . while carrying a naked merchant banker . . ." She hesitated. "Do you think if we close our eyes and say . . . we don't believe in stairs, then maybe . . ."

She went up them almost crouching under the weight, desperate to put down her burden and rest for a while, certain that if she did they'd never make it.

When they reached the corridor, the doorway was still visible, still moving steadily away. Maria said, "Run ahead and . . . keep it open."

"How?"

"I don't know. Go and stand in the middle . . ."

Durham looked dubious, but he limped forward and reached the doorway well ahead of her. He stepped right through, then turned and stood with one foot on either side, reaching out a hand to her, ready to drag her onto the departing train. She had a vision of him, bisected, one half flopping bloodily into each world.

She said, "I hope this . . . bastard was a great . . . philanthropist. He'd better . . . have been a fucking . . . saint."

She looked to the side of the doorway. The corridor's dead end was only centimeters away. Durham must have read the expression on her face; he retreated into the control room. The doorway touched the wall, then vanished. Maria bellowed with frustration, and dropped Riemann onto the carpet.

She ran to the wall and pounded on it, then sank to her knees. She was going to die here, inside a stranger's imploding fantasy. She pressed her face against the cool paintwork. There was another Maria, back in the old world -- and whatever else happened, at least she'd saved Francesco. If this insane dream ended, it ended.

Someone put a hand on her shoulder. She twisted around in shock, pulling a muscle in her neck. It was Durham.