It's been a week since Peter created the bridge between universes on Liberty Island - and then disappeared from reality. Inside the neutral zone, the two Olivia Dunhams trade case files and trash talk. Olivia says that Bolivia's side can't be trusted. Bolivia snipes that Olivia is incapable of trust.
As Olivia turns away, a man appears and disappears behind her in a blurry flash. Peter?
Two Observers meet in a diner. September says that the timeline has been rewritten, yet "traces of him continue to bleed through." December says that September started this, so he has to end it.
The Agent Lincoln Lee of This Side arrives at the home of his partner, Robert Danzig, where he jokes with Robert's wife and kids. Clearly, he's one of the family.
Shortly later, after capturing a suspect, Lincoln finds his partner down. A man is leaning over him - a man with a translucent face. The stranger flees, but Robert's dead. His face has become translucent too.
At the scene, Olivia introduces herself to Lincoln. Watching nearby, another Observer tells September that Lincoln used to know Olivia. Yes, says September, before things changed. Olivia says that her special division is taking Danzig's body. She brushes off Lincoln's indignant questions, but he gets her license plate number when she leaves.
In his Harvard lab, an agitated Walter tells Astrid that THAT MAN cannot be trusted! Astrid says Walternate has plenty of reasons to hate Walter too. Gene the cow moos. Meanwhile, Olivia looks at Danzig's case file, then pulls out the file on John Scott, her long-dead partner.
Lincoln tracks Olivia to the lab and walks straight into the weird world of Fringe Division. Olivia threatens to report him if he doesn't leave. Lincoln threatens to call his friend at The New York Times, who would love to know about whatever all this is, if she makes him go. Stalemate.
Broyles calls about another body, found after a car accident. Olivia grudgingly takes Lincoln along. Victim Nadine Park's skin is translucent, like Danzig's. Astrid calls Walter at the lab and uses her new wireless camera earpiece to show him the body. Meanwhile, Lincoln impresses Olivia with a hunch that scores them a witness's cellphone picture of the translucent man.
Olivia explains that the freaky see-through killer has more than two victims. Lincoln gets a higher clearance and agrees not to blab about what he sees. In a room full of bodies, Broyles says that the first attacks happened three days ago. There must be a link, but what? Lincoln notes that Robert took iron pills for his Crohn's disease. Hmm.
Lincoln sadly tells Olivia that being Robert's partner for five years gave him something he never thought he'd find: a place to belong.
In a cluttered electronics shop, September gathers parts from the shopkeeper. What's he building? A device to erase someone from time.
As Nadine's body arrives at Harvard, Walter emerges from the isolation tank, dripping wet and panicky. He hid because he saw a man in the mirror, but when he turned around the man wasn't there. Astrid thinks that Walter means Agent Lee, but Walter shouts no, ANOTHER man was here, and then gone. Walter SAW HIM! Olivia calms him down.
Privately, Olivia assures Lincoln that Walter is mostly functional and often brilliant. He's just never had anything to tether him to the world.
Walter says Lincoln's idea to test the victims for iron-related diseases was clever, but wrong. However, the 30-odd victims all had health problems caused by poisoning from heavy metals, like mercury. The metal levels in their blood are normal now, as though their blood was cleaned. He doesn't know what it means.
In a lab somewhere, the translucent man gives himself an injection and makes strange notations in a book.
Lincoln is outraged that the victims' bodies won't be returned to their families. Olivia explains that would draw undue attention to Fringe Division. Sorry.
The team realizes that the killer is operating around four commuter rail stations. While waiting for a lead, Olivia confides to Lincoln that she too lost a partner, and explains about Agent Scott. They track down the killer, who burns his files and brutally attacks Olivia before she shoots him. Lincoln kills a second translucent man . . . who looks like Danzig. What are these things? Unseen nearby, a third one turns into Nadine Park.
To Lincoln's surprise, Olivia has Robert's body released. At the lab, she says she's never known the sense of belonging he mentioned, but if she had . . . She trails off. She hopes this brings him closure.
Walter, who's examining the translucent creatures, holds up a small computer disk. Shapeshifters! But these are different; they're human. And for whatever reason, they don't work. Walter KNEW that man couldn't be trusted! Lincoln asks what he's talking about.
Olivia ushers Lincoln into the neutral zone, saying sometimes answers lead to more questions. He stares through the windows at the zeppelins on the Other Side. Olivia hands Bolivia the disk, explaining it was powering a new form of human shapeshifters. Olivia says that the ones they encountered before answered to Walternate. Bolivia will look into it.
Inside the lab, Walter bids good night to the agent watching him. Outside, September prepares to activate his device, then stops and walks away. Walter cries out: Peter's reflected in his TV! Frightened, he smashes the screen. He tells the agent he saw the man from the mirror again. Walter hugs himself, terrified.