At Etta's apartment, Olivia naps while Peter finds a hidden arsenal in the wall. Olivia awakens and questions why they would get her back - just to lose her again?
In Manhattan, some Observers assemble a cube-shaped piece of tech and use it to open a shipping portal from the future, transporting components for the air-degradation system in Central Park.
Olivia boxes up Etta's things, and Walter keeps her favorite perfume, because his sense of smell helps him remember. Etta's phone rings inside the box. It's Anil, who needs the Fringe team's help.
Outside Central Park, Anil explains about the shipment, and Walter realizes the Observers have created a wormhole. The resistance wants to destroy the next shipment. They've captured an Observer and have his notebook, but they can't read the language. Maybe Astrid can decrypt it.
The captured Observer also had a cube, which Peter determines is a powerful energy source that can help them destroy the corridor. Everyone protests: They don't know how the cube works. But Peter's determined. Shortly later, Anil, referencing Confucius, tells Peter, "Before you go on a journey of revenge, dig two graves." Peter brushes him off.
At Harvard, Walter tells Peter it's possible, though dangerous, to destroy the corridor. He demonstrates how to turn the wormhole into a black hole by collapsing their end, using the cube and an antimatter baton. But the cube must be put together properly. Peter resolves to make the captured Observer tell him how.
Walter overhears Olivia trying to talk Peter out of his plan. Later, Walter says he knows she's afraid they'll lose each other again. He gives her a videotape of one of Etta's birthdays and urges her to watch it with Peter, to remember what they both had, what they still are, and hold onto that. They must face this pain together: It's proof that Etta was here. And you can't escape it by building walls around your heart, by breaking a universe, or by vengeance. But Olivia can't watch it.
Anil tells Peter that Observers' abilities are aided by tech. He points out a chip in the back of their captive's neck.
Peter verbally spars with the Observer as he uses a close-up video of the Observer's eye to guide his cube assemblage. If the Observer's pupil dilates, Peter will know he's doing it right. If it contracts, he's doing it wrong.
Astrid decodes the Observer's notebook. The next shipment is this afternoon. Peter's plan briefly appears to work - but then they quickly realize it has failed. How?
As Peter rushes away, Olivia sees posters in an alley: Etta's face with the word "Resist."
Peter attacks the Observer, who recites the Confucius quote . . . and reveals that Observers can't be read through pupil movements. Peter only saw what he wanted to see because his emotions give meaning to things that aren't there. Peter rages that he'd be ten times what the Observer is if he just had that tech in his head.
Peter removes the Observer's chip . . . and inserts it into his own neck. Just afterward, Olivia - who watched the birthday videotape - calls. She tearfully says she wants him to come home. Etta would want them to be together, to survive this. She loves him. Peter says he loves her too, then hangs up. He stares at himself in the mirror as the chip takes hold.