Olivia and Fauxlivia are trading files from their two Fringe Divisions and Olivia complains that they can’t trust their counterparts. Fauxlivia notes that Olivia might have trust issues, and Olivia reminds her that she kidnapped her and stole her life. When Fauxlivia comments that Olivia has no life, Olivia tells her that she knows nothing about her and they finish exchanging files. As they leave, Peter briefly flickers into existence and disappears.
The Observer, September, meets with his fellow Observer August at a diner. August tells September that they have a problem, and September insists that Peter has been eliminated from the timeline. However, he admit that bit of Peter’s existence are bleeding through. August reminds him that the Observers are sworn to ensure events play out as they were intended, and that September must finish it because it started with him. He hands him a small electronic component and tells September that they can never know that the boy grew to be a man. September says that he’ll take care of it.
In Hartford, CT, FBI agent Lincoln Lee arrives at the home of his partner, Robert Danzig. Robert is getting ready for work and his son and daughter run to greet Lincoln. His wife Jules comes in and tells her children to head to school with her while Robert finishes getting dressed.
Later, Robert and Lincoln are chasing a suspect across the Hartford rooftops. They follow him into a building and split up to corner the suspect, and he gets the drop on Lincoln. Lincoln disarms and handcuffs him and then calls for his partner. He gets no response and doubles back, and finds a translucent-skinned man standing over Robert’s body. The attacker leaps out a window and Lincoln goes to his partner, who is death with his face similarly distorted.
Olivia and Astrid are called to the scene and check on Robert’s body where it’s being kept in an ambulance. Astrid is wearing an earpiece and checks with Walter, while Olivia talks to Lincoln. Nearby, September and another Observer watch, and the other Observer notes that Lincoln used to know Olivia. Meanwhile, Lincoln tells Olivia what happened. When he realizes that Olivia isn’t going to identify who she’s with, and that she’s taking Robert’s body with her, Lincoln objects but Olivia expresses her condolences and leaves. As she goes, Lincoln gets the license plate of her car.
At Harvard, Walter complains that there’s something different in the lab’s environment since they’ve agreed to work with the Fringe Division on the Other Side. Walter says that they can’t trust Walternate and Astrid reminds him that Walternate has reason to hate him as well. Olivia is working in her office and going over the files on Robert. Lincoln comes in and Walter hands him a seemingly dead bird and injects it with fluid, bringing it back to life. As the bird falls back down, dead, Olivia comes out and demands to know why Lincoln is there. He explains that he tracked her via traffic cams and Olivia warns him that he has to leave because he’s not part of the investigation. Lincoln refuses until he gets answers and demands to know what they’re dealing with. When Olivia threatens to call his superior, he threatens to call the New York Times
. Before one of them cracks, Olivia gets a call that there’s been another dead body. Lincoln demands to go saying there may have been something at the first crime scene that he didn’t report. She games to let him come with her and Astrid.
At the new crime scene, the local police have found a 30-year-old dead woman with the same translucent skin. She died in a car crash and nobody saw anything beforehand. Meanwhile, Astrid calls Walter, who tells her to show him the body over the earpiece’s camera. He spots a hematoma on her jaw, suggesting premature rigor mortis, and tells Astrid to check the body’s anus. Meanwhile, Olivia confirms that the dead woman was Nadine Park, a commuter. She parked her car at the car station that morning and Olivia figure the killer was in the back seat. He killed Nadine, she struggled, and the car hit the lamppost. Lincoln notices that one woman is sitting by herself and Olivia goes over to talk to her. The woman says that her name is Olivia and claims she didn’t see anything. Once Olivia promises that she won’t get in trouble, the other Olivia says that she was in the street talking to her boyfriend on her cell phone, and took a picture of the killer. As they leave, Olivia congratulates Lincoln and he says that he could do more if he had more information. After a moment, Olivia tells him that there have been more than two victims.
At FBI headquarters, Broyles meets with Lincoln and gives him paperwork confirming that his clearance has been raised and that he can now be prosecuted for revealing anything he knows. Broyles then takes him to a room with over thirty corpses. The first one appeared three days ago and there’s no indication of how they died. There’s no common link between any of the victims. Lincoln notes that Robert had Crohn’s disease and took iron pills. Once Broyles leaves, Lincoln looks at his partner’s body and tells Olivia that he was like family. He explains that Robert believed that everything happened for a reason, but he doesn’t know what the reason is for the deaths.
The next day, September goes to a used parts shop, touches one part, and it sparks. As he takes it, the owner comes over and says that he’s found the electrode gun that September requested. When he asks what September needs the parts for, the Observer says that he needs to erase someone from time.
FBI agents bring Nadine’s body to the lab and Astrid tells Olivia that Walter has disappeared even though he’s afraid to leave the lab. They start searching for him while Walter emerges from the sensory deprivation tank, much to Lincoln’s surprise. Walter says that there’s a man in his mirror who keeps disappearing when he turns around to look at him. Astrid and Olivia return and try to reassure him. As Astrid goes to get him some clothes, Olivia tells Lincoln that Walter is functional but never had anything to tether him to the world.
A near-naked Walter examines Nadine’s corpse and notes that she has an engagement ring, and that it’s sad when two people are meant to be together and something intervenes. The test results print out and Walter informs Lincoln that all of the victims suffer from health ailments caused by heavy-metal poisoning. He doesn’t know what it means but explains that all of the heavy metals have been removed from their bodies... and the killer needs a lot of them.
In a hidden lab, the killer prepares a syringe and injects himself with a yellow fluid. He then writes in a journal.
Olivia calls Broyles with an update and tries to work out how the killer identifies his victims. Lincoln realizes that don’t plan to return the bodies and Olivia explains that they inform the families that the victims have simply gone missing. He’s shocked that they lie to the families and asks Olivia if she can imagine that kind of hole in her life. Walter tells him that people die, and sometimes they die twice. Olivia explains that turning over the bodies would mean revealing Fringe Division’s existence. As they talk, Astrid picks up a charge on one of the victim’s credit cards but then realizes it’s a monthly charge on the victim’s parking pass at the commuter station. Olivia checks and confirms that the killer strikes in four areas near the commuter station. She takes Lincoln with her while Atrid calls Broyles.
As he writes in his notebook, the killer starts to shake. He studies an implant in his arm and watches as his fingernails grow out. He pulls one out, and then after a moment he studies his face in a mirror and smiles triumphantly.
Olivia and Lincoln arrive at the Avon Commuter Station in Massachusetts and coordinate with the FBI teams watching the entrances. As they sit in the car, Olivia tells Lincoln that she does know what it’s like and explains that she lost a partner about three years ago and he died in a horrific manner. She checked out Walter, who was in an institution, but Walter was unable to save her partner. Agent Roach calls to notify them that they’ve spotted the killer heading for a warehouse. Roach and Frazier go in on foot and the kller grabs Frazier and drags him beneath a train. When Lincoln and Olivia arrive, they find Frazier dead and Roach shot. Lincoln tells Olivia to find the killer while he tends to Roach.
Olivia goes into the warehouse after the translucent man and hears him moving around. He starts a fire with his bare hands and then starts burning files from his briefcase. Olivia finds him and tells him not to move, but he hits her with the case and then grabs her. She stabs him with a syringe but he shrugs it off and throws her into some barrels. She finally manages to grab her gun and shoot him.
Lincoln hears the gun shots. He looks in a window and sees that Olivia is okay, and then goes back to Roach and makes sure that he’s okay. The agent says that there’s more than one killer and Lincoln sees another one moving between the trains. He goes after the killer, hears him around a corner, and spins around. They shoot simultaneously and Lincoln kills his target.
That night, the EMTs take the bodies away and Lincoln asks Olivia who the kiler s are and what did they want. Olivia says a better question is what they are. Meanwhile, a woman with the same translucent skin watches them from an oil tower. She concentrates and her skin turns solid.
Later, Lincoln goes to the Hartford FBI office and collects Robert’s belongings, including a certificate.
At the lab, Astrid analyzes the burned papers but has no luck. Lincoln arrives and admits to Walter that he doesn’t have any candy. He thanks Olivia for pulling some strings and releasing Robert’s body, and wonders why she would do that. Olivia says that he hasn’t found all the answers and hopes that she could provide some closure for Lincoln. Walter autopsies one of the translucent men and finds a biological-mechanical hybrid, but it doesn’t appear to be work. He insists that the tech isn’t from their world and insists that Walternate is involved.
Olivia takes Lincoln to a secure facility and has his physical statistics confirmed. They then enter a room and wait while they’re decontaminated. As they wait, Olivia explains that after her partner’s death, she joined Fringe Division knowing she would find some answers. The door opens and Olivia warns Lincoln that sometimes answers just lead to more questions. They step forward into the chamber with the Machine. Fauxilivia comes in, much to Lincoln’s astonishment, and Olivia gives her the hybrid device used to power human shapeshifters. Olivia says that’s why she doesn’t trust her Other side counterparts, and Fauxlivia says that she’ll look into it. As she walks away, Lincoln looks up and sees an Other Side airship fly by.
Outside the lab, September arrives with a piece of electronic equipment. He starts to activate it but then reconsiders, shuts it down, and walks away. Inside, Walter goes to his office to sleep and watches the TV. He finally turns off the TV but sees Peter’s image in the darkened screen. He screams and the security guard, Timmy runs in as Walter insists he saw the man in the mirror.