Sarah told herself that a wolf might kill a deer, but never would it be murder. She told herself that. As a doctor, though, she was committed to human welfare, and that certainly did not include killing. The creature beside her had eaten children and fathers and mothers—had eaten them. As had she herself . . . in the shame of her secret life.

And yet, this was the agony of it: nature uses predators to ensure balance. One reason that human overpopulation was destroying the world was that the Keepers had failed in their natural mission. There were not enough of them to make a difference.

Miriam called herself part of the justice of the earth. And Sarah could not deny that. She had looked into the teary eyes of victims, seen them fade as she did her own clumsy sucking. She had known what it was to be engorged with human blood. Afterward, you felt as light as air. Any small imperfection disappeared. You became supple. Your skin regained a girl’s flush and milk. And your heart—it beat with a happiness that seemed founded in something that was deeply right. You had dared the abyss, to do the bidding of nature. What an addiction it was, the addiction to death.

Sarah knew that she was using her strange new relationship to the laws of nature to justify herself. But she had not been given a choice. Miriam had fallen in love with her and had infused her own blood into Sarah’s veins without her permission, putting her to sleep to do it. Sarah had awakened exhausted, aching in every bone, not knowing what had happened.

There had begun an awful struggle. She had tried to live on blood bought from commercial blood banks. She had tried to live on animal blood. Then she had refused to live at all. She had actually died and been put in a coffin and slid in among Miriam’s other expended lovers in her attic.

But Miriam had used Sarah’s own research to bring her back. Sarah had eaten, then. She had not been strong enough to return to the terror of the coffin. Because, when Keeper blood flowed in human veins, you could live for centuries, but you could never really die.

Sarah had experienced the silent, trapped sensation of being unable to move, to breathe, to so much as flutter an eyelid in that coffin. She had been aware of the dark around her, of the lid above her, of the rustle of insects along her skin and the murmur of street traffic outside.

She’d heard Miriam playing her viola, had heard jets passing overhead, had heard the lapping mutinies of the East River and the hiss of the FDR Drive. She gone mad a hundred times, mad in the locked-up remains of her body. All around her, there had been other such coffins, some of them thousands of years old, that contained other trapped souls.