A man, BOB DUNN, prepares for a night out on the town while watching the evening news. The reporter says there's been a gruesome murder, whose details haven't yet been released to the public.
Later, as Dunn arrives in the Boston industrial district, he calls a WOMAN, his girlfriend. She's out of town, and there's a hint of jealousy about what Dunn might be doing in her absence. Dunn says he has a boring business meeting -- then enters a nightclub.
He hits on one girl, but with no luck. Then, from across the dance floor, he makes eye contact with a woman with striking, bright blue eyes. Later, in his apartment, the two of them are making out. The woman takes his head between her hands and without warning snaps his neck! Later, the woman looks at herself in the bathroom mirror -- while a dead Dunn lies facedown on the bed with his spinal column jutting horrifically from his neck.
Olivia's apartment. Olivia and Rachel are having a dinner party with friends. In the middle of the party, Rachel receives news that her husband Greg is filing for divorce. Before Olivia can properly console her, she gets a call from Charlie summoning her to a crime scene.
The crime scene at Bob Dunn's apartment. His girlfriend found him dead when she returned from out of town. The gruesome results match those of the other recent killings that were reported on the news. Walter thinks the bite marks on the neck are human, but he also indicates that they were made by something with above-normal human strength.
At the FBI office, Olivia meets with Broyles. Dunn's girlfriend said his car was missing, so maybe the killer is using it. Before leaving, Olivia asks Broyles if he was happy with his divorce attorney's name. Broyles says he was satisfied with everything but the bill and gives Olivia the lawyer's information.
The Harvard lab. Walter, in doing the autopsy, has found that Dunn's body was completely drained of spinal fluid. Moreover, there were traces of syphilis bacteria on his exposed spine -- an extinct strain of the bacteria, probably from killer's saliva.
Olivia meets with an official from the CDC, who says a sample of the extinct bacteria was shipped to Luboff Pharmaceuticals in Brighton. The CDC makes samples of pathogens available to reputable institutions for scientific research. But Luboff has an address in a residential neighborhood. In addition, Luboff has requested many contagions over the years, including RUD 390 -- a compound used in bioweapons, including the skin-growth toxin used by the ZFT terrorist organization ("Ability").
An FBI strike force enters the Brighton address of Luboff Pharmaceuticals. Inside they find a man, NICHOLAS BOONE, seated in a wheelchair while working on a dead animal's spine.
At the FBI, Olivia and Charlie interrogate Boone. They ask what he knows about ZFT? Boone knows a lot about ZFT, and he's willing to reveal it, but only if the FBI first helps find his wife. ZFT kidnapped her, and they're holding her as punishment for Boone trying to leave the organization.
An FBI team prepares to raid a Chinese restaurant where Boone says his wife is being held. Olivia gets a call from a distraught Rachel, who says Greg is seeking custody of their daughter. During the raid, the team finds a laboratory hidden beneath the restaurant, but Boone's wife is not there. Olivia talks on the phone with Boone, who wants her to acquire a dose of the contagion inside a refrigerator in the lab. He needs it to make an antidote. They didn't kidnap his wife, he says; they infected her as a punishment. Boone's wife is the killer.
While inspecting Boone's laboratory, Peter finds a video camera with a tape of Boone and his wife in happier times.
Meanwhile, VALERIE BOONE, the killer, has found a new victim. The guy notices she's burning up, almost like she has a fever. She says she's sorry -- and then opens her mouth wide like a snake and attacks.
Later, Charlie finds the new victim in Roxbury. At the FBI, Olivia shows Boone the video Peter found. The video shows clearly that Boone is not wheelchair-bound, even though the video was only a short time ago. Boone explains that the contagion causes Valerie's body to burn through her cerebrospinal fluid, and she kills to refuel. He fed her from his spinal fluid until he couldn't anymore.
He tried to synthesize an antidote, but he needed a dose of the bacteria to work from. Syphilis, he says, was just a platform -- a carrier for other attributes. But Olivia asks why? ZFT wanted the super syphilis for the same reason they wanted the skin-growth toxin: to show off to other scientists. Boone says his offer still stands: he will tell them everything he knows about ZFT, on the condition that they find his wife. In the meantime, he will work on the cure.
Boone is taken to the Harvard lab, where Walter shows him around. Charlie says they've found Bob Dunn's missing Audi, stripped down on a street in Mission Hill. The GPS is missing. Peter has a friend, Mako, who runs a chop shop, they will go talk to him. Meanwhile, Walter and Boone have come up with a plan: they will use super-penicillin to kill super-syphilis.
At Mako's chop shop, Mako says the GPS has been wiped clean of all information, but the car was boosted on Route 18, near Weymouth, by the water.
Back at the lab, Boone and Walter have infected a rat with the contagion. It seems this strain of syphilis raises body temperature substantially. Also, the cure is nearly complete. Walter is impressed by Boone's education and intellect, and it turns out Boone knows who Walter is. They have a conversation about whether there is any such thing as a soul, whether there are things man shouldn't know, and whether they will be judged one day for their scientific trespasses. Walter says he and "Belly" -- William Bell, the founder of Massive Dynamic and richest man in the world -- used to have similar debates when they shared this laboratory.
Route 18 in Weymouth. Olivia and Peter find shattered glass by the road. Apparently Valerie killed Dunn, and then took his car. In the woods nearby, they find more victims.
At the lab, Walter and Boone inject the rat with the antidote to see if it will cure it, while capturing their work on camera. Olivia wonders where Valerie is finding her victims. Astrid notices that one of the victims smells of alcohol, and she says to turn off the lights. Walter claps to turn off the lights (he's installed a clapper!), and Astrid finds a glowing entry stamp on two victims' hands for the Cavern, a popular destination in the industrial club scene. When they turn the lights back on, they see the rat is dead. They need more time to perfect the cure. Boone begs the agents to bring his wife in alive.
The Cavern. Valerie is on the hunt, while Olivia and Peter search for her. Peter is looking at the crowd with a thermal camera, hoping to identify someone with elevated body temperature.
In the lab, Boone realizes why the cure wasn't working. They need cerebrospinal fluid as a catalyst for both the contagion and the cure. And, he says, his spinal fluid is the only kind they know for sure is compatible. Walter says it's too dangerous, because taking more fluid could cause a brain hemorrhage, but Boone insists that he has enough left.
They perform the spinal tap. After the procedure, Boone looks at the video camera; he's clearly thinking of something.
Back at the club, Peter finds a hot spot, but then loses her. Valerie is leaving the club with a new victim when Charlie shoots her with a tranquilizer gun.
At the lab, Boone starts shaking. Walter realizes that Boone lied; he didn't have enough cerebrospinal fluid left to give. Why? Boone replies, "How far would you go for someone you love?"
Olivia and Peter are driving Valerie to the lab, when she suddenly wakes up in the back seat. As Peter and Olivia talk about Rachel and whether cynics are romantics in disguises, Valerie attacks. Peter is forced to again shoot her with the tranquilizer gun.
They bring Valerie into the lab and strap her down. In the meantime, Boone has had a stroke. They inject the cure directly into Valerie's spine. She seizes, screams, pulls against her restraints... then goes slack. Walter pulls back her eyelid to reveal her eye changing from blue to green. The antidote appears to be working. But Olivia notices that Boone has died. Olivia's face shows her disappointment: there went her chance to learn about ZFT.
As Valerie is wheeled out of the lab, Walter approaches with video tape, which Boone made before he died. They insert it into the VCR, and Boone begins to speak. He says they made a bargain, and Olivia help up her end. He expresses the hope that the information he gives them will help to rectify some of his mistakes. Although he never dealt with anyone of sufficient rank in the organization, he managed to gather some names -- including some they might recognize.
Olivia meets with Broyles at a restaurant. She says Boone never mentioned the name David Robert Jones (a known member of ZFT). But he did mention the name of the man funding ZFT... William Bell.