In a dark laboratory, a man with a tube in the back of his brain looks at a photograph of a young boy. Another man asks how he felt on the day that picture was taken. The man's breath condenses in the air, and he says, "Happy." His eyes freeze over.
At Harvard, Walter tells Lincoln about the shapeshifters, then starts ranting about the horrible Other Side people while obsessively covering up all the reflective surfaces. Astrid admonishes Walter, who drags Lincoln away to help cover a mirror in another room. Olivia arrives and good-naturedly deflects Astrid's suggestion that she ask Lincoln out.
Broyles calls about a new case involving 23 victims, showing the same MO: a hole drilled at the back of the skull, and a chemical pumped in that lowers the brain's temperature. The victims froze from the inside out. Why hasn't Olivia heard of this? Because it didn't happen Over Here. The Other Side has asked for assistance.
Broyles and Olivia meet with Bolivia, who explains that the suspect, John McClennan, is a highly intelligent psychopath. Bolivia wants Olivia to bring their world's McClennan to the Other Side, in hopes that he can help them learn more. Olivia objects, and Bolivia says that she's already been through this with her own Colonel Broyles . . . who's still alive, unlike before Peter created the bridge between universes and then vanished. It's one of several things that have changed on both sides.
This Side's John McClennan is a respectable professor whose specialty is serial killers. This just might work. Professor McClennan agrees to do the profiling and to be sedated during transport so he won't know where he is.
At the killer's house in Norristown, Pennsylvania, Bolivia puts on a blonde wig and banters with Agent Lincoln Lee, saying that Frank, her boyfriend, likes her red hair. Olivia arrives with the still-unconscious professor. She'll monitor things remotely with Agent Lee.
At Harvard, Walter plays Mozart at a foundation-rattling volume. Astrid takes off the record, scolding him. Realizing he's deeply troubled, she asks what's wrong. He says that he just needs to rest.
The professor sees some egg-hatching lights in the killer's house and tells Bolivia he recognizes them, because his dad worked on a farm. He spots a chair that's exactly like one from the house where he grew up. Weird. Looking at a display of photos taken from the victims, he says that the killer hates that they have happy lives. He takes people when they're happy, because he wants to feel better.
When the professor finds a photo of his own father, however, he becomes angry and confused. Storming out, he's suddenly between two identical Olivias. Dazed, he asks, "Where am I?"
Olivia explains about the two universes, saying people make different choices and therefore end up in different places. Seriously rattled, the professor says he too has darkness in him. His father saw it - and used a heavy hand to try to fix him. Now Bolivia's listening too. Olivia tells him that she had an abusive stepfather. McClennan says a woman named Marjorie taught him that when he got a dark urge, he could just step out of it, into the light. He wishes he could tell the killer that there's another way. Olivia says no way.
Olivia goes outside, where Agent Lee's briefing Bolivia about a woman who's just been kidnapped by the killer. Meanwhile, the professor escapes out the bathroom window.
At Fringe HQ, a fuming Colonel Broyles asks for leads. Olivia recalls the plate number of the tractor next to McClennan's father in the photograph, leading them to the old farm.
The killer's working on the woman when the professor arrives. He lunges for a toolbox, which the professor predicts holds a .45 pistol, because that's where his dad kept it too. The professor explains he was like the killer, until one night in October, when he was ten. His father found the dead things and came after him. The killer says, "That's MY life." But the killer's father caught him and beat him for three days. The professor ran, and ended up with Marjorie. She helped him step out of the darkness - and now he can help the killer. Maybe you can, says the killer, and knocks the professor out.
En route to the farm, Bolivia asks Olivia what happened to her stepfather. "I killed him," Olivia replies.
The killer uses his brain-freezing machine to steal the professor's memories of Marjorie. Olivia and Agent Lee arrive, and she confronts the killer. He says he wanted what Marjorie gave the professor, but now he feels remorse. What has he done? He kills himself with his father's gun.
Back Over Here, Olivia visits the professor in the hospital. He doesn't remember the case . . . or Marjorie. But he tells Olivia that, even when things are darkest, we can step into the light. Later, Olivia marvels that he knows what Marjorie taught him, but not who she is. How is that possible? Broyles thinks some people leave an imprint on your soul that can never be erased.
At Harvard, Walter gets into bed. All the reflective surfaces are covered, so he won't see the man from the mirror again. Then Peter's voice fills his head. Over and over, he says, "Walter, I'm here; can you hear me?" Walter freaks out. "Help me, Walter. Please help me."