The girls began to cry in earnest now, Sophie burying her face in Lily’s shoulder.

“Nathan, if they cut the backup, we lose the seals on the doors,” I said.

He did not answer immediately, his mind clearly racing through the schematics of the house.

“The air vents have mechanical overrides,” he said, walking to the tool chest in the corner. “But the main power lines run through the service closet in the hall.”

“We can’t go out there,” I said. “Julian has a gun.”

“I have to,” Nathan said, pulling a heavy steel wrench from the drawer. “If I don’t rewire the lock from the inside panel, they’ll starve us of oxygen in thirty minutes.”

He looked at me, his eyes filled with a desperate, quiet intensity.

“You have to stay with the girls,” he said. “If they get through, take them through the crawlspace behind the safe.”

“The crawlspace leads to the generator room,” I said. “It’s freezing in there.”

“It’s better than here,” he replied.

Before I could answer, the amber lights went completely black.