“What do you want, Lydia?” I said, avoiding Stellan’s gaze. We drove over the river. The Paris streets were even more empty than yesterday, only a scattering of people waiting to cross at the light in front of Notre-Dame.

   “You’re welcome for not hurting your sister, Avery’s boyfriend. I’m not the monster my sister says I am.”

   I don’t know what I’d expected Stellan to say, but it wasn’t a quiet “Thank you.”

   “So I’m calling to tell you,” Lydia said, “that this is your fault. Things were going to be fine, but by turning the Circle on us, you’ve ruined everything. Now I have to clean it up, and that annoys me. I hate having to resort to threats. It’s so crass.”

   “Threats?” I said. Anya whimpered and pulled her hand away. Stellan must have squeezed it.

   Lydia sighed. “The only way to ensure our family’s safety—and therefore the whole Circle’s, like I tried to tell you—is to have the Circle behind us. They were getting there rather well on their own, but now that’s changed and we’ve got to give it a push. Since we know this virus infects anyone with a trace of Circle blood, we could wipe out half a city with a snap of our fingers. And the Circle knows it. So now, if they don’t cooperate, their people pay.”

   Stellan made a strangled noise. “Lydia—” I said.

   “The Circle will see eventually that this was the only way. They’ll thank us. But until then, you’ve made us be the bad guys, and I really wish you hadn’t.”

   “Lydia! You don’t have to—”

   “Good-bye, Avery.”

   • • •

   “How are they doing it?” I said the second we reunited with Jack and Elodie. I tossed my bag onto the table in the Dauphins’ sitting room and threw myself into a chair.

   The Saxons had made their threat to the Circle. Bow to their authority, or have your territory obliterated by the virus. So much as a hint of interference from us or anyone else and they’d pull the trigger immediately.

   “I suppose they’ll make up some kind of treaty that everyone will have to sign, giving them full control over the Circle. They’ve called a meeting for tomorrow evening, giving the families a day to get everything together. They haven’t said where the meeting is yet, except that it’s somewhere in Europe,” Elodie finished before I could ask.

   “There’s no way the Circle will sign that,” I said. The Circle only cared about the Circle. About their own family, in particular, and their power. They wouldn’t give it up to save the lives of regular civilians.

   No one argued with me.

   I squinted at the morning sun through the window. We’d been in Russia and flying all through the night. At least we’d gotten a few hours of sleep on the plane. I rubbed my forehead. I couldn’t stop thinking about talking to Lydia. How does a girl—someone so much like me—turn into this?

   Stellan came back into the room. The Dauphins’ nanny was still here, caring for Luc’s orphaned baby brother, and Stellan had taken a few minutes to get his sister set up in another one of the rooms in the nursery.

   “So if we can’t stop the release of the virus,” I said, “we’ll have to stop the virus from killing people. We have the vaccine. How fast can we get it out?”

   While we were in Russia, the science team had confirmed that the theory about my blood being a vaccine was true.

   “The problem is scale,” Elodie said. “Because of its regenerative properties, a single drop of the virus diluted in millions of gallons of liquid would still be lethal.”

   “Like if they were to put it in a city’s water supply,” I said.

   Elodie nodded. “Or aerosolize it. For the vaccine, though? It takes a drop of your blood per person. Even if we drained you dry, it wouldn’t be enough to vaccinate a single Paris neighborhood, much less the whole world.”

   I pushed up my sleeves. It suddenly felt way too hot in here.

   “Um, excuse me.” We turned to find Nisha standing in the doorway. She and the entire science team had been based at the Order headquarters here in Paris, but while Stellan and I were gone, Elodie had moved them to the Dauphins’ so we’d all be in one place. “I’m sorry to interrupt. We have an idea.”

   • • •

   The lab tables and microscopes looked out of place in the Dauphins’ ornate dining rooms. On one of the tables, Nisha and the rest of the scientists were crowded around a Plexiglas box with a tiny white mouse sniffing at its corners. I leaned down and touched my finger to the glass. The little mouse nosed at it.

   “As you know, we had been attempting to deactivate the virus,” Nisha said in her soft accent as we gathered around, “but that has proved impossible so far—and now that the Saxons have your blood, that avenue is closed for the moment anyway. When we learned the cure was really a vaccine, though, we wondered whether we could use the same mechanism of the Great modification in Mr. Korolev’s blood to make the vaccine more effective. It looks promising.”

   Elodie twisted one of her small gold earrings. “Promising how?”

   “We are using the Great modification in a specific way to make only the right parts of the mouse’s cells replicate quickly.”

   Elodie was the first to understand. “I thought you were experimenting with altering Avery’s blood. Like, in a vial.”

   “Unfortunately,” Nisha said steadily, “it would be impossible to modify the blood once it’s already outside the body.”

   All the curtains were drawn, and it was dim enough that it could have been any time of day as we looked at each other over the makeshift chemistry lab. “I don’t understand,” Stellan said. “You said my blood takes the tiny bit of virus in her blood and multiplies it so fast it becomes deadly. Wouldn’t this just trigger that same process inside her?”