After the surprising kiss, Peter asks why Olivia did that. She doesn't know; it just felt normal. He says that Walter should look at her, but Olivia insists she's fine, just a little embarrassed. Also, she feels another migraine coming on. As Peter leaves, she has a flashback to them in the original timeline.
Three young men murder a journalist named Daniel Greene in Douglaston, New York - while a schizophrenic patient hears their voices and pantomimes the crime in an institution in Deerfield, Massachusetts. Nurse Bernadette sedates the patient, Sean Keenan. The next morning, she sees news about the murder. Enter the Fringe team.
Bernadette tells Olivia, Walter, Lincoln, and Astrid that Sean's never behaved like this. Olivia has a flashback to when she and Peter retrieved Walter from St. Claire's. Sean tells Walter that the voices come and go. They don't speak to him, only to each other. He recognizes eight, maybe nine. He heard three last night. Before the sedative kicked in, one said he was cut and needed to clean up in the bathroom before getting on the train.
Walter says Sean isn't schizophrenic; he's hearing the real thoughts of other people. Sean agrees to go off his meds to help them. Olivia has another flashback to St. Claire's.
Later, Olivia visits Peter and asks if she can look around. Inside, she flashes back again, but she's serene, like everything's clicked. She remembers William Bell, crossing over to bring Peter back, Jacksonville, and seeing him shimmer the first time they kissed. She remembers him walking to the Machine, and being scared he was going to die. Peter says something must be wrong. Olivia says that she remembers everything.
Walter examines Olivia, who also still remembers the non-Peter timeline, but it's hazy. Citing her strong sense of empathy, Walter suspects that she's accommodating Peter's desire to be reunited with his Olivia by becoming her. He takes a hair sample.
Lincoln brings in a DNA report on some bloody paper towels from the Roosevelt Street Station bathroom. Walter notices that Sean and the suspect have the same incredibly rare abnormality. They must be siblings. But he's an only child.
Olivia and Lincoln leave to interview Sean's mother. Walter suspiciously lectures Peter, who insists that he's not doing anything to Olivia. Walter says to him that he is, even without meaning to.
Sean's mom says he was conceived through in vitro fertilization, with donor sperm. So he could indeed have a half-brother. Her doctor was Owen Frank. Also, a reporter named Daniel Greene left a message a couple of days ago, about a piece he was doing on IVF.
Dr. Frank now resides in an assisted-living facility, where he watches a TV report about Greene's murder.
Sean tells Astrid he can hear the voices again. He can't tell what they're saying; there are too many of them.
Back at the lab, Olivia and Peter struggle with her revelations. Walter interrupts with his latest breakthrough, involving animals that use nonverbal communication. If Sean and his half-brothers are linked in a telepathic network, they may have formed a collective identity.
Olivia and Peter interview Dr. Frank, who says Greene visited him last week . . . and "they" have killed before. Three years ago, a writer he hired was stabbed to death before they could start documenting his work. The doctor thought it was random, but now he's sure it wasn't.
His experiments attempted to make a better human being through recombinant DNA - mixing genetic material from different species. He wanted to reintroduce abilities humans have evolved away from. Like telepathy? Yes. And a heightened protective instinct? Yes, they're killing to protect themselves. If his work went public, people would want to study them. His clinic's files are at a storage facility. All 200 subjects are from the same donor: him. He marvels at his hubris in trying to improve upon God.
Walter looks at a printout and says, "Good God!" Lincoln arrives, and Walter insists they go see Nina Sharp.
Walter angrily informs Nina that Olivia has Cortexiphan in her system. Someone must have synthesized it, but how? Nina asks how Olivia is. The Cortexiphan samples he and Bell kept are at Massive Dynamic, in a vault only she can open.
As Olivia and Peter look for the storage unit, they talk about how hard this is. She just wants him to behave naturally. She mentions the storage-unit explosion with John Scott, saying it was rigged with Semtex - a detail Peter hadn't known. Walter's wrong: Peter can't be projecting memories onto Olivia.
Tipped off by Sean, Astrid calls Olivia and says the killers know they're there. The killers attack, but Peter and Olivia win. However, it's too late to stop Dr. Frank's murder.
Olivia and Peter stop at a gas station. What now? Peter's afraid, because he made this mistake before: He betrayed the Olivia he loved. But what he's really afraid of is, when he looks in her eyes, he knows it's her. They kiss. She laughs, and he says what? She just has to, uh, go pee.
Nina shows Walter and Lincoln that all 20 Cortexiphan samples are there. Now, what's happening to Olivia? Walter sips from a vial and announces it's not Cortexiphan. It's potassium iodide, with food coloring.
Peter looks for Olivia in the gas station, but she's disappeared.
In a murky basement room, Olivia comes to, disheveled and tied to a chair. Nina asks if they hurt her. "It's OK," says Nina, who's also disheveled and tied to a chair. "We're gonna be OK."