Dr. Kato greedily eats from a wrapper on the ground in the trashed hallway...then sees a microwave parked in the middle of the hall. It's on, with a live rat inside. Sure enough, it's bait: when she goes for a closer look, two vectors grab her. Peter, covered in ropey veins, watches smugly. The rat explodes.
A phone buzzes on the desk where Hatake is taking a nap. It's Sutton's phone, and because she hasn't checked in, Ilaria is "initiating retrieval protocol." A six-hour countdown starts ticking away. Hatake removes a case from a safe; it's filled with explosives, which Hatake activates.
Jules is also napping, next to Sarah's bed. She starts to rise when Alan enters, but he urges her to rest; she's just had a spinal tap. Alan adds that, according to the MRI, Sarah's tumor is shrinking, so evidently he injected Sarah with Jules's spinal fluid. Jules also wants to use her spinal fluid to save everyone else on the base who's infected. Alan doesn't think she should, but she insists, and eventually he agrees to set up a lab -- but reminds her Ilaria is coming eventually, and who knows how much time they'll have. Dr. Duchamp runs in to report that people have started disappearing.
While Hatake sets explosive charges around the base, Alan and Duchamp inspect the microwave. Duchamp sighs that they warned Kato not to go alone; Alan thinks the rat indicates the vectors have returned, knowing food is scarce and people will venture out...only to fall into their traps. Duchamp thinks this is virologically impossible; The virus has evolved, and Duchamp wonders what chance the healthy folks have now.
Hatake lets Alan know Ilaria's forces are en route, determined to repossess the virus (which Alan mistakenly believes they destroyed). Then they argue over whether Hatake is full of it when he claims he's immortal. Enter Aerov with a head count: roughly 20 uninfected people, plus five security guys. Alan wants them herded into the sunroom and guarded; Aerov reminds him that they can barely fend off the "coordinated vector attack" Alan's predicting, and Ilaria is going to overrun them in less than six hours. Hatake's plan -- lure Ilaria's forces into the base, then implode it on top of them and the vectors -- is the only workable one. The uninfected will be evacuated...to a hidden bunker below the base that only Hatake knew existed.
In the sunroom, Duchamp is not feeling Alan's plan. Alan hopes their distress call got out, but he needs Duchamp's help holding "this" together. Duchamp reluctantly assents. Above them, in a grate, Peter and another vector regard Alan hungrily.
Aerov is offended that he's only now finding out about the bunker, and doesn't understand how they'll dig out after the blasts. An escape tunnel leads to the surface, Hatake says. Aerov is skeptical. Hatake can't do it without him. Aerov glares.
Jules checks to make sure Sarah's still unconscious, then takes the contraband Narvik out of her coat and starts testing. Downstairs, Aerov stockpiles weapons and keeps bitching at Hatake, who grits out that Aerov's loathing of him "does nothing to diminish" his parental feelings for Aerov.
As the vectors do mysterious things with tools and Q-Tips, Jules reports on her RNA study. In a nutshell, she can program the Narvik to destroy itself. That's the good news. The bad news is, Alan is RILLY MAD she held onto the virus, thereby endangering Earth's population: "You're no better than Hatake!" That gets to Jules; tearfully, she swears she's right about her theory; if she's not, she'll incinerate the remaining virus. "Not alone you won't," a distrustful Alan says, and BOOM Sarah hits the glass between her and the two of them like a crazy bird. Elsewhere, the vectors haul Dr. Kato to a table and drain her of her black blood.
Turns out the bunker is the Montana-cabin replica from Jules's childhood. It'll just barely fit all of them, Hatake says, before confessing that, before he adopted Aerov, "there was another" -- Jules. Aerov glares, then rants that everything that's happened "was all for her," and Hatake can't deny that he needs Jules. Aerov bitterly notes that everyone's an experiment to Hatake, and stomps off.
With the vectors on the march, Sarah asks to help with the vaccine. Alan isn't having it, but she's obviously good as new intellectually, as she brainstorms that the issue is how fast the virus moves -- and that they could try slowing it down with cryofluid to give the antiviral time to work.
Meanwhile, the vectors rig up a fire alarm so the sprinklers, which they've filled with black blood, will go off and drench the sunroom. Cheery music plays as the healthy freak out and try to escape, in vain.
Alan surveys the empty sunroom, then reports back: except for four security techs and the main characters, everyone else is now infected. Alan wants to find Duchamp and the others, but Aerov and Hatake feel their only hope is the bunker. Alan protests that that means leaving human beings behind; Hatake wonders how much humanity remains in them, and they nearly come to blows after Hatake says he should have "put" Peter "down" when he had the chance.
Then it's time to test Sarah's hypothesis, which will involve injecting the cryosolution directly into vectors' stomachs. It seems to work on the rat they inject; the next step is to freeze the entire base again by cutting the power so the vectors don't kill them, but Ilaria's only a couple hours away and Aerov doesn't like the idea.
But he has an alternative: he crafted a Ghostbusters proton-pack-like contraption that allows them to spray cryofluid on the vectors, then get close enough to inject them. Hatake is stunned and proud, then volunteers the two immortals, himself and Jules, for vaccination duty; Alan's to lead the survivors to the bunker when Ilaria lands.
In their vironaut suits, Jules and Hatake run across their first vector: Peter. After he's hit with the freeze, Jules injects him with the antiviral, and he hits the deck spewing black goo, then passes out. She's pleased it's working, but brushes off Hatake's compliment: "This is not father-daughter bonding time." And here comes a squadron of angry vectors to avenge Peter...
...but Jules and Hatake dispatch them with ease. Alas, others have eluded them and taken to the air vents.
Sarah's grouped the patients so the sickest ones go to the bunker first...but she's stricken by another headache. Her scans are clear, but Alan's concerned.
Jules approaches Peter's bedside. "Why me?" he gasps; he doesn't deserve it. "You were there, I had the cure," she smiles. He looks pointedly over at Alan. Jules goes to tell Alan Peter's resting; he's more concerned that Sarah's symptoms have recurred. The PA announces a perimeter breach; Ilaria has arrived.
Jules rounds up the patients for the bunker; Hatake estimates Ilaria's 50 miles away. He, Alan, and Aerov head outside to mark Ilaria's approach; a hundred snowmobiles crest a rise, they shoulder their machine guns, and Aerov warns that they can't waste ammo, so let them get close. The snowmobiles cut the lights and stop, so the plan is reformulated: head around to the south, downwind, and cut them off. But when they come upon the empty snowmobiles, they realize the vehicles are drones -- rigged to explode. One of them kabooms and Aerov tackles Hatake to the ground, but not before Hatake gets singed. "It's a diversion," Alan shouts -- as paratroopers parachute onto the base and blow the door.
In the bunker, Hatake whispers that "he's" here: the Scythe, an assassin Hatake calls "the worst of us." When the Scythe and his attendants come upon Duchamp and the others in the elevator, Duchamp assumes they're CDC and invites them into the elevator, burbling that they're rescued and cured. Behind his back, the Scythe holds, well, two scythes, and it's no surprise that when the doors open again, it's only the three black-clad Ilarians left standing amid blood spatters.
Aerov wants to blow the base; Alan wants to wait for the others. Sarah has a migraine attack, and when she looks up, her eyes flash silver. Aerov takes advantage of the distraction to grab the detonator, and Alan nods, but the charges don't go off when Aerov squeezes the trigger. Outside, a Scythette pulls a charge off the wall and snorts, "Amateurs," and the Scythe grunts, "The Narvik is gone." He orders the Scythettes to find Hatake; he'll have the virus. And the others? The Scythe removes his helmet to reveal a teenage boy with silver eyes, who shrugs, "Exterminate."