The path grew narrower, running horizontally to the mountain’s face as it descended, clinging to it like a catwalk. To his left, sheer rock, glinting with moonlit ice; to his right, an abyss of blackness, a plunge into nothing. Even to look at it was to be swept away; he kept his eyes forward. The women were moving quickly, shadowy presences leaping at the far edge of his vision. Where was Lacey taking them? What was the weapon she had spoken of? He could hear the voice of the river again, far below. The stars shone hard and pure above his face, like chips of ice.

He turned a corner and stopped; Lacey and Amy were standing before a wide, pipelike opening in the mountain’s face. The hole was as tall as he was, its depthless interior a maw of blackness.

“This way,” said Lacey.

Two steps, three steps, four; the darkness enveloped him. Lacey was taking them inside the mountain. He remembered the tin of matches in his coat. He stopped and struck one, his insensate fingers fumbling in the cold, but as soon as it sparked, the swirling currents of air puffed the flame away.

Lacey’s voice, from up ahead: “Hurry, Peter.”

He inched his way forward, each step an act of faith. Then he felt a hand on his arm, a firm pressure. Amy.

“Stop.”

He couldn’t see anything at all. Despite the cold he had begun to sweat under his parka. Where was Lacey? He had spun around, searching for the opening to orient himself, when from behind him came a squeal of metal, and the sound of an opening door.

Everything blazed with light.

They were in a long hallway, carved from the mountain. The walls were lined with pipes and metal conduits. Lacey was standing at a breaker panel on the wall adjacent to the entrance. The room was illuminated by a bank of buzzing fluorescent lights, high above.

“There’s power?”

“Batteries. The doctor showed me how.”

“No batteries could last this long.”

“These are… different.”

Lacey swung the heavy door closed behind them.

“He called it Level Five. I will show you. Please come.”

The hallway led to a wider space, sunk in darkness. Lacey moved along the wall to find the switch. Through the soles of his wet boots he could feel a kind of humming, distinctly mechanical.

The lights buzzed and flickered to life.

The room appeared to be some kind of infirmary. An air of abandonment hung over all-the gurney and the long, tall counter covered with dusty equipment, burners and beakers and chrome basins, tarnished with age; a tray of syringes, still sealed in plastic, and resting on a long, rust-stained shawl of fabric, a line of metal probes and scalpels. At the back of the room, in a nest of conduits, was what appeared to be a battery stack.

If you found her, bring her here.

Here, Peter thought. Not just the mountain, but here. This room.

What was here?

Lacey had stepped to a steel case, like a wardrobe, bolted to the wall. On its face was a handle and, beside this, a keypad. He watched as the woman punched in a long series of numbers, then turned the handle with a thunk.

He thought at first the case was empty. Then he saw, resting on the bottom shelf, a metal box. Lacey removed it and passed it to him.

The box, small enough to fit in one hand, was surprisingly light. It appeared to have no seams at all, but there was a latch, with a tiny button beside it that perfectly fit his thumb. Peter pressed it; at once the box separated into two perfectly formed halves. Inside, cradled in foam, lay two rows of tiny glass vials, containing a shimmering green liquid. He counted eleven; a twelfth compartment was empty.

“It is the last virus,” said Lacey. “The one he gave to Amy. He made it from her blood.”

He searched her face to see the truth registered there. But he already knew the truth; more than that, he felt the truth.

“The empty one. That’s you, isn’t it? The one Lear gave you.”

Lacey nodded. “I believe that it is.”

He closed the lid, which sealed with a solid click. He slid off his backpack and pulled out a blanket, which he used to wrap the box, then placed it all inside. From the counter he retrieved a handful of the sealed syringes and put these in the pack as well. Their best chance was to make it through till dawn, then get down the mountain. After that, he didn’t know. He turned to Amy.

“How long do we have?”

She shook her head: not long. “He’s close.”

“Can he get through that door, Lacey?”

The woman said nothing.

“Lacey?”

“It is my hope that he will,” she said.

They were in the field now, high above the river. Peter’s and Amy’s trail had disappeared, covered by the blowing snow. Alicia had ridden ahead. It should have been dawn by now, thought Michael. But all he saw was the same gray softening they’d been riding toward for what seemed like hours.

“So where the hell are they?” said Hollis.

Michael didn’t know if he meant Peter and Amy or the virals. The thought occurred to him, with a vague acceptance, that they were all going to die up here, that none of them would ever leave this frozen, barren place. Sara and Greer were silent-thinking the same thing, Michael thought, or maybe they were just too cold to speak. His hands were so stiff he doubted he could fire, much less reload, his rifle. He tried to take a drink from his canteen to steady himself, but it was frozen solid.

From out of the darkness they heard the sound of Alicia’s horse, riding back at a trot. She pulled up beside them.

“Tracks,” she said, gesturing with a quick tip of her head. “There’s an opening in the fence.”

She heeled her mount, not waiting for them, and barreled back they way she’d come. Without a word Greer followed, the others bringing up the rear. They were in the trees again. Alicia was riding faster now, galloping through the snow. Michael heeled his mount, urging the animal forward. Beside him, Sara bent her neck low over her mount as the branches skimmed past.

Something was moving above them, in the trees.

Michael lifted his face in time to hear a gun go off behind him. No sooner had this happened than a violent force slapped him from the rear, shoving the air from his lungs and catapulting him headfirst over the horse’s neck, his rifle swinging out from his hand like a whip. For a single instant he felt himself suspended painlessly over the earth-part of his mind paused to register this surprising fact-but the sensation didn’t last; he hit the ground with a jolt, landing on his back in the snow, and now there were other things to think about. He had, he saw, come to rest directly in the path of his own horse. He rolled over on his side, covering the back of his head with his hands as if this might actually help; he felt the wild torrent of air as the panicked animal bounded over him, followed by the concussion of its hooves, one impacting just inches from his ear.

Then it was gone. Everyone was gone.

Michael saw the viral-the same one, he surmised, that had knocked him from his horse-as soon as he drew up to his knees. It was crouched just a few meters from him, poised on its folded haunches like a frog. Its forearms were buried in the snow, which glowed with the organic light of its bioluminescence, as if the creature were partially immersed in a pool of blue-green water. More snow clung to its chest and arms, a glistening dust; rivulets of moisture were running down its face. Michael realized that he was hearing gunshots, an echoing spatter over the ridge and, mixed with this, like the words of a song, voices calling his name. But these sounds might have been signals from a distant star. Like the vast expanse of darkness around him-for that, too, had faded from his mind, dispersing like the molecules of an expanding gas-they might have pertained to some other person entirely. The viral was clicking now, rocking the muscles of its jaw. With a cock of its head it gave a lazy-seeming snap of teeth, as if it were in no hurry-as if the two of them had all the time in the world. And in that moment Michael realized that the place where he kept his fear was empty. He, Michael the Circuit, wasn’t afraid. What he felt was more like anger-a huge, weary irritation, such as he might have felt for a fly that had been buzzing around his face too long. Goddamnit, he thought, guiding his hand to the sheath on his belt. I am so tired of these fucking things. Maybe there are forty million of you and maybe there aren’t. In the next two seconds, there’s going to be one less.