I stared at her. “We’re tapeworms,” I said. “We’re parasites.”

“Yes. You don’t kill your hosts on purpose, although you’re more than happy to rewire them to suit your needs. Humans, on the other hand, have a long tradition of killing our hosts. It’s almost a genetic imperative with us.”

“But we kill the original personality,” I protested.

“Biology doesn’t care. The genes are still there; the body is still alive,” said Dr. Cale. “I’m not saying Sherman had the right idea by encouraging his people to increase the amount of human DNA in the implants—if it was Sherman; I’ll be able to tell whether he asked them to use any of my research techniques once I’ve had the chance to cross-check this data against the recent specimens that Tansy brought back from Lafayette—but I am saying I understand how he could have talked other people into going along with him.”

“If this was Sherman, how is he activating the sleepwalkers?” asked Nathan. “What mechanism is he using?”

“I don’t know yet,” said Dr. Cale. “But I will. Trust me on that. I will.”

“My head hurts,” I said, putting one hand against my temple. It felt surprisingly fragile now, like I was carrying around my brain and my body in an eggshell, one that could smash open and spill me on the floor at any moment. “I want to go home.”

Dr. Cale actually looked alarmed. “I’m sorry, but I can’t let you do that.”

“What?” I lowered my hand, staring at her. “What do you mean, you can’t let me do that? I want to go home. Why is that so difficult?”

“Because SymboGen’s security is going to be looking for you by now, and Tansy still isn’t back,” said Dr. Cale. “It’s not safe for you to go out without someone to keep an eye on you. The sleepwalker activity in San Francisco is rather extreme right now.” She shot a meaningful look to Nathan, who frowned and looked away.

I scowled at the both of them. “You didn’t mention my—I mean, Sally’s father, who’s probably looking for me by now, especially if that treatment we suggested for Joyce didn’t work, and did I mention that I don’t care what you think? I want to go home. Beverly and Minnie need to be let out, or they’re going to destroy the apartment.”

“Beverly and Minnie?” asked Dr. Cale blankly.

“Our dogs,” said Nathan. “Sal’s right, Mom. Even if it’s not safe for us to stay in the apartment anymore, we can’t leave the dogs alone there. They’ll run out of food and water, and I’m not willing to do that to them.”

Beverly and Minnie were rescues, casualties of the same epidemic that had claimed so many human lives. Beverly’s owner was last seen in the hospital, sunk deep in the coma that claimed many sleepwalkers in the early stages of their illness. He’d been hospitalized when he lost consciousness, and he’d still been there when vital services began to collapse. I didn’t know whether he had died or woken up and shambled off to join his fellows, but either way, I wasn’t giving back his dog. Minnie’s situation was similarly tragic, and made slightly worse by the fact that Nathan and I had known her owners. Katherine had become sick and had killed her wife, Devi, who used to work with Nathan at San Francisco City Hospital.

Whether she was still trapped in the hospital or loose on the streets of San Francisco, I hoped that Katherine didn’t remember anything about the woman she used to be: I hoped her colonizing worm had wiped her original identity cleanly away. No one should have to live remembering that they murdered their own wife.

I shook my head vigorously. “I didn’t come here to be your prisoner, and I’m not going to stay if I’m not allowed to come and go when I want. I’ve already been through that with the Mitchells. If you’re not willing to let me go and take care of my dogs, I’m going to leave anyway, and I’m not going to come back.”

Dr. Cale sighed, sagging in her chair. “Do you understand that this is all about your well-being?” she asked. “It’s not safe out there, and from what I’ve been able to determine so far from the data you retrieved from SymboGen, it’s not going to get safer anytime soon. Things are going to get much worse before they get better.”

“How much worse?” asked Nathan.

“Ten percent human DNA,” said Dr. Cale grimly. “That’s an apocalypse number.”

“How soon?” asked Nathan.

Dr. Cale hesitated before she said, “I don’t know.”

“Then why don’t we compromise?” I asked. They both turned to me. “Nathan and I will go and get the dogs. We’ll be careful, and we won’t let ourselves get arrested or eaten or anything, but we’ll go, because we have to go. And then we’ll come back here. You won’t keep us prisoner, and we’ll be here willingly for as long as it takes to find an answer that doesn’t end up in the extinction of the human race. But you have to show that you can let us go before we’re going to agree to do that. You have to show that you’re playing fair.”

“I don’t see—” began Dr. Cale.

“Mom,” said Nathan. His voice was soft, but it stopped her dead. She looked at him for a moment, tilting her chin up to compensate for the difference in their heights. Then she sighed.

“I don’t like this,” she said. “You’re my son, and she’s virtually my daughter, in more ways than one, and I don’t like this at all. I’m supposed to keep you safe. It’s my job. How am I supposed to be a good mother to you if you won’t let me do my job?”