“We’d be using what we’ve learned about the modification, not necessarily your blood, as it were. And we are attempting to isolate certain parts of the cells. Small distinctions, but important. The hope is that it will affect only the amount of vaccine in the blood, and not turn into the virus and kill . . . the mouse.”

   I felt everyone’s gaze on me.

   “No.” Stellan stepped in front of me like he might block me from what they were thinking. “You’re not using me to try to turn her into a walking vaccination clinic. Not using what we think might be some science from thousands of years ago and that we’ve studied for a few days.”

   I glanced out the window to an eerily empty Louvre courtyard. “The vaccine’s the only chance we have left, isn’t it?”

   No one answered.

   “The serum is ready,” said another of the scientists, approaching the box with a full syringe. I’d seen her at the Order meeting. Half of the Order crew we’d seen at the meeting was here, in fact.

   “This is a trial to see whether this mouse can get the vaccine in its blood?” I said.

   “Not exactly. The science is complicated. You would be the only one able to grow the vaccine in your blood, but these trials give us a better idea of whether the virus will take hold instead. If you wanted to go through with this, we would do trials until we found a version of the serum that did not kill the mouse, and then . . .”

   Then try it on me. And either it would work and we’d have a way to distribute the vaccine—or it wouldn’t and I’d die of the virus myself.

   “Show me,” I said.

   Nisha nodded. “Subject T-twenty-three. Commencing trial of substance two-point-six,” she said into a handheld recorder. She picked up the mouse and injected it with the syringe’s contents.

   Stellan gripped the Plexiglas. Across from us, Jack and Elodie peered in, too.

   When Nisha set it back down, the mouse shook itself, then darted across the box. Darted back. After a couple of minutes, Nisha dropped in a lettuce leaf, and the mouse ran to it, nibbling at the edge. “If it’s feeling well enough to eat, that’s a good sign,” she whispered excitedly.

   “How long would it take for it to get infected?” Stellan asked.

   “It happens quickly in the mice. We’ll have to watch it for a few hours, but this is very promising—”

   A bump on the side of the box drew everyone’s attention back down. The mouse was stumbling. There was a bead of blood coming out of its ear, bright against its white fur. The other two scientists hurried over. As we watched, the mouse fell on its side, hemorrhaging and convulsing, blood trickling from its mouth. It didn’t move again.

   Everyone’s eyes flicked to me.

   “I’m sorry,” Nisha said. “We thought we had it.”

   I pulled my hair back into a ponytail, snapping a few strands of hair caught in one of the new bandages. “Do you need more of my blood for the experiments?” I said, forcing a smile.

   • • •

   We gathered in the parlor again.

   “No,” Jack said without preamble. “Absolutely not.”

   It felt like the moments right before a plane took off. You’re hurtling along the ground faster and faster, so fast you can feel what has to happen next. Things were careening toward an end.

   “Even if they find something that doesn’t kill the mouse, we can’t inject you until it’s been thoroughly evaluated,” Stellan added. “Any drug goes through years of trials before it’s tested on humans.”

   Jack nodded. For once, the two of them were in agreement.

   I watched the dust swirling in the morning light through the window. The curtains in here were wide open. “We’ve followed the clues for a long time, and this is where they’ve led. If we were supposed to go to a new place, or solve a new puzzle, we’d do it.”

   “But this is not—” Jack started.

   I cut him off. “I could have taken myself out of the equation to stop this earlier, and now the whole world is in danger. It might not be my fault, but it’s my blood. I have to seriously consider it.”

   “And if it doesn’t work, it’s my blood that could—” Stellan couldn’t seem to say the words.

   Luc had gone to make some calls, and now he came back into the room, Rocco on his heels. As soon as we’d left Russia, we’d given word to Rocco that his undercover days in the Saxon household were over. Lydia would know Omar hadn’t worked alone. “There may be another factor to consider,” Luc said. “I just heard that over half the families have accepted the terms and plan to sign the treaty.”

   “What?” We seemed to all say it at once.

   “I was surprised as well,” Luc said. “It’s not all of the families yet, but many of them are giving up their power for their people’s lives.”

   That was something I hadn’t expected.

   “That’s it, then,” Stellan said. “If the whole world isn’t going to die, we let the Circle fend for themselves. We could leave some of your blood with the scientists, and eventually they may develop a viable vaccine. Or not, and the Saxons run out of the virus at some point. And we wash our hands of it. We leave. We go to some secluded island and—” He met my eyes. “We go to two different secluded islands. On opposite sides of the world so even if they find one of us, they won’t be able to weaponize us. And that’s it. That’s the end.”

   “But if they sign the treaty and give all that power to the Saxons . . .” I said.

   “The Saxons, for lack of a better term, rule the world,” Jack finished.

   I twisted my locket. “If we had a vaccine, we might be able to get it out before they sign. Keep half the world from dying and keep the Saxons from becoming dictators.”