Olivia and Fauxlivia argue about how hard it's going to be to trust each other. They swap files from their respective worlds.
The Observer joins another, who tells him they have a problem. Traces of Peter are bleeding through. The Observer is still trying to set things back how they were supposed to be before he intervened to save Peter as a child. "They can never know the boy lived to be a man," Observer Two says. The Observer will take care of it.
Lincoln Lee goes to pick up his partner Robert, saying hi to his wife Jules and two kids.
Cut to them racing on rooftops after a bad guy, all holding guns. They follow the bad guy into a warehouse of dark rooms and tunnels. The bad guy sneaks up on Lincoln, but he quickly subdues him. Lincoln calls for Robert but gets no answer. He looks through the warehouse and finds a disfigured man kneeling over his partner's body. The man looks like Robert. He takes off, leaping with super-human strength. Lincoln looks at Robert and finds his face is disfigured as well, like it's changing shape.
Olivia and Astrid respond to the scene. The Observer and another watch Olivia talk to Lincoln. Lincoln describes the man's translucent skin. Olivia asks if the man was holding anything. They're taking the body. Lincoln wants answers but Olivia isn't permitted to give any. He takes note of her license plate as she drives away.
In the lab, Walter complains that something feels different.
Lincoln walks into the lab and Walter gives him a dead dove to hold while he injects it with something. The bird flies off. Walter explains it's still dead and then it suddenly drops to the floor, dead again. Olivia demands to know what Lincoln is doing there.
He tapped into traffic cameras to find them. He refuses to leave. He wants to know what division they're in. He threatens to tell the "The New York Times" about them. They're interrupted by Broyles calling to say there's another body. Lincoln tags along.
The victim is a 30-year-old woman. Her face is translucent and moldy. Astrid calls Walter on her earpiece/camera, which gets feedback when Walter microwaves popcorn. He sees burn marks and wants to check closer.
The victim is Nadine Park. Lincoln notices one of the witnesses sitting off on her own, looking quietly freaked out. Olivia goes to talk to the witness, who's name is also Olivia. She took a picture off a translucent man at the scene. Olivia confesses to Lincoln that there have been more than two murders.
She takes him to see Broyles, who has forms for him to sign swearing him to secrecy. They go into a room with dozens of bodies that all died the same way in the past three days. They haven't been able to find a common link. Lincoln offers up that Robert had Crohn's disease and took iron pills.
Robert was Lincoln's partner for five years, they were like family.
The Observer looks through an old parts store. He finds a switch he likes, and the owner gets him something from an old ray gun. He asks what it's for. "I need to erase someone from time," he says.
In the lab, Walter has disappeared. Lincoln finds him in the LSD tub, hiding. Walter freaks out, saying there was a man there and then he was gone. Olivia explains to Lincoln that Walter never had anything to tether him to the world.
Later, Walter stands over the latest victim. Walter gets the test results about iron levels, which show nothing, but he did find the victims all had health problems related to heavy-metal poisoning, like from mercury.
The level of metals in their bodies has returned to normal, like someone cleaned their blood. Walter theorizes the man needs a lot of what he's taking.
Cut to a man with translucent skin injecting himself with something. He makes a notation.
In the lab, Walter cuts open the victim. Lincoln is aghast to learn that they don't plan to tell their families they're dead because they can't say how they died. He asks if they can imagine what it would be like to have a hole in their lives like that. Walter says everybody dies, some people die twice.
Olivia says the death documentation would lead to Fringe division and they can't have that.
Astrid finds a charge on a victim's card -- a commuter pass being renewed. They look at the commuter lot map and find the victims are coming from off the lines. They head out.
The translucent man tries to make a notation but his blood is acting up under his skin. One of his fingernails comes off. He appears to be happy about this.
Three teams, including Lincoln and Olivia, stake out the commuter lots. Olivia tells Lincoln she lost a partner three years ago. She tells him she checked Walter out of a mental institution to try to save her partner's life, but he couldn't.
They get a call from a team about the translucent man. Two agents follow him through a rail yard. He grabs one of them. Olivia and Lincoln arrive to find him translucent and the other man shot.
Olivia follows the man into a warehouse. Upstairs, he burns his research papers and packs up. Olivia finds him, but he whacks her with his briefcase and throws her around his lab like a rag doll. She manages to get her gun back and empties it into him.
Out with the wounded agent in the rail yard, Lincoln hears the shots andclimbs a car to look into the warehouse. He's relieved to see she's still standing. The wounded agent tells Lee the man took his gun. And there's more than one.
Lincoln sees someone run by and follows. He comes upon him around a railcar and fires. It's his old partner.
As they take away the dead later, another watches from nearby. She looks like Nadine Park.
FBI HQ, Harford
Lincoln packs up his partner's things. He gets an envelope with a certificate inside.
In the lab, Astrid looks over the charred notes. Walter says the situation reminds him of "The Spy Who Came in From the Cold". Lincoln came to thank Olivia for having Robert's body released.
Walter finds a metal circuit in the latest. He knows the technology isn't from here. He thinks it proves Walternate can't be trusted.
Olivia gets Lincoln a badge so he can enter the portal. Olivia tells Lincoln she's had a hole in her life for a long time and she joined Fringe to find answers.
He asks what's beyond the door.
"Sometimes answers lead to more questions," she says. She shows him the Doomsday machine and the window to the two worlds.
Fauxlivia joins them. Olivia shows her the tech that was powering a new form of human shape-shifters. Olivia points out the ones they encountered before answered to Walternate.
The Observer reads his machine outside Walter's lab.
Walter tucks himself in to bed inside.
The observer powers up his machine with some sadness. He can't bring himself to flip the switch. He powers it down and walks away.
Walter turns off his TV but screams when he sees a face. It's Peter. The guard rushes in and Walter says he saw the man from the mirror again.