“Stop!” Now she could hear him. “Stop where you are.”

Instead she dropped to her stomach and slid beneath the gap, shimmying her hips free when for a moment the bottom of the fence snagged on her pants. The guard was still shouting at her to stop but she was out, out and free and once again helping Cassiopeia to her feet. She didn’t know why she was so afraid, but she was. At any second she expected to hear the chitter of bullets on the fence, feel her heart explode sideways, cleaved in two by a bullet.

But the shots didn’t come, although the guard was still shouting, still coming toward them. At that second there was another rocketing blast (the fire had found its way to the storerooms in the basement of B-Wing, stocked with old chemical samples, medications, solutions marked flammable and dangerous), a final explosion that shot a plume of green flame fifty feet into the air and made the ground shudder. Cassiopeia slipped and fell backward in the mud. Lyra stumbled, and 72 caught her. For a few seconds they were inches apart, and she could smell him again, and see the fine dark line around his irises, light contracting his pupils, narrowing them to pinpoints.

From above came hailstones of granite and cement, several of them lobbing over the fence and thudding only a few feet from where they were standing. The guard had dropped to his knees and covered his head, and Lyra saw their chance. Together, she and 72 hauled Cassiopeia to her feet and went with her into the marsh. Lyra wasn’t sure what they were going to do about Cassiopeia. Already she regretted taking her along. But Cassiopeia was number 6. Like Lyra, she was Gen-3, the first successful crop. Lyra had known her for as long as she could remember.

The water was warmer than she had expected, and cloudy with dirt. Banks of waist-high grass grew between stretches of thick mud and tidal pools scummy with dead insects, all of it new and strange to her, words and feelings she didn’t know, sensations that tasted like blood in her mouth and panic reaching up to throttle her. Several years ago, the replicas had been woken by screaming: a man a half-mile from Spruce Island had his leg ripped off at the hip by an alligator before the guards scared it by firing into the air. He was airlifted to a nearby hospital. The nurses had for once allowed them out of their beds to watch the helicopter land with a noise like the giant whirring of insect wings, white grasses flattened by the artificial wind. Once, when she was a child, she’d even seen an alligator sunning itself on the rocky beach on the southernmost tip of the island, not four feet from the fences. She had been amazed by its knobby hide, its elongated snout, the teeth protruding jaggedly from its mouth, and she remembered standing there flooded with sudden shame: God had made that creature, that monster with a taste for blood, and loved it. But he had not made her.

She felt as if they were walking through endless tunnels bound entirely by mud and grass, and couldn’t imagine that 72 knew where he was going, or where he was leading them. Cassiopeia was crying, and only the smoke still lodged in Lyra’s chest, still turning the sun to a dull red ember and smudging away the sky, kept Lyra from crying, too. Haven, gone. They were outside the fence. They were in thin, unbound air, in a world of alligators and humans who hated and despised them. They were running away from safety and Lyra didn’t know why. Only that the guard had come at her with a gun, looking as if he wanted to shoot.

Why had he drawn his gun? The guards were there for their own protection. To keep the outside world out. To keep the replicas safe.

The mosquitoes, at least, had been chased off by the smoke, although no-see-ums were still hovering in swarms over the water, and Lyra got some in her nose and mouth and even beneath her eyelids. From here, the sound of the fire was strangely musical and sounded like the steady roar of a heavy rain. But the sky was green-tinged and terrible, and the ash floated down on them.

Her arms were shaking from trying to keep Cassiopeia on her feet. Even the pillowcase felt impossibly heavy. Cassiopeia was clinging so tightly to her neck, Lyra could hardly breathe. Cassiopeia was passing in and out of consciousness, and Lyra imagined her mind like a series of ever-branching tunnels, like the marshland crisscrossed by fine veins of water, going dark and then light again.

“How much farther?” Speaking hurt.

72 just shook his head. She knew that human men were in general stronger than women and wondered whether the same thing was true of replicas. He looked strong—the muscles of his back and shoulders stood out—though he couldn’t have been eating well since he escaped. She wondered where he’d gotten his food. She wondered why he’d been so desperate to get out, and whether he knew something she didn’t. Or maybe he was just crazy—plenty of replicas had lost their minds before, like how Lilac Springs had lost her mind during her examinations, had forgotten all the numbers she was supposed to remember. There was Pepper, who’d used a knife to open her wrists, and number 220, who’d simply stopped eating, and number 35, who’d started believing she was one of the rats and would only crawl on all fours. Maybe 72 was like that. Maybe he believed he was an animal and should roam free.

She couldn’t go on anymore. Cassiopeia was too heavy. Every breath felt like it was hitching on a giant hook in her chest. She tried to call out to 72 but realized she didn’t have the energy even for that. Instead she struggled with Cassiopeia into the reeds, finding footing on the muddy banks that stretched like fingers through the water, until the ground solidified and she could sit. 72 had to double back when he realized she was no longer behind him.

“We aren’t safe here,” 72 said. He didn’t sound like he’d lost his mind. She noticed how dark his eyes were, so they appeared to absorb light instead of reflecting it. “I should leave you,” he said after a minute.