Back at Etta's apartment, Peter looks at a photo of Etta and compares it to a sleeping Olivia. He then goes through Etta's belongings and finds a hidden compartment behind a painting. Inside of it are C4 packs. As Peter puts them in a duffel bag, Olivia wakes up and admits that she keeps thinking that she's experiencing a nightmare. She wonders why they would get Etta back just to lose her and cries as Peter hugs her. Later, Olivia goes into the bathroom and notices the family photos that her daughter kept... and starts taking them down.
Out on the streets of Manhattan, people notice as bursts of electricity start coming down. A loyalist convoy pulls up and two men take a case into position while Observers watch and record everything. One Observer removes a cubical device from the case, activates it, and sets it down. A portal opens and three large white cubes are sent through, the energy discharge charring the entire street. The Observers cut open a cube and scan the interior, confirming that integrity is stable and telling the loyalists that they are ready for transport.
As Olivia packs away Etta's belongings, Peter comes out to get Walter that it's time to go. Walter wants to keep a bottle of Etta's perfume so he'll have an odor to help him remember her. Peter agrees and gets a call from Astrid from the lab. She warns them that the next tape is near a propane tank and it will take her extra time to cut through the amber. Etta's phone ring and Olivia answers. It's Etta's partner in the underground, Anil, and he asks that they meet.
Peter, Olivia, and Walter go to the street where the transport arrived and Alin pulls up in a van. He offers his condolences and says that he hopes someday they'll understand how important Etta was. Alin then explains that the Observers opened a wormhole from the future to bring back components for the air-degradation system they're building in Central Park. Alin warns that if the Observes finish the machine then the atmosphere will undergo full conversion. The resistance wants to destroy the next shipment when it comes and Alin shows them a journal that they took from one of the Observers, written in their own language.
When Olivia wonders where they got it, Anil calls another resistance fighter, Niemann, and has him show the others that they have a captive Observer at a secure location. Peter notices burn patterns in the shape of a cube on the ground and Alin confirms that the Observer had tech with him. They figure the device was used to create the wormhole and Peter suggests that they figure out how to use it to destroy the entire corridor. Anil warns that they're not familiar with the technology and Walter agrees, but Peter insists that they should go ahead with his plan.
Walter prepares to collect some samples and Olivia goes to the van to get Anil's kit. Meanwhile, Anil tells Peter that he should prepare two graves if he's seeking revenge, but Peter says that he's not worried about being destroyed and asks them to bring the cube. After a moment, Anil calls Niemann and tells him to bring the control cube.
Once they have the cube, Peter takes it back to Harvard and tries to work out how it functions. Meanwhile, Astrid tries to decode the journal using a computer program and admits she isn't having much luck. When she notices that Olivia is in shock, Astrid assures her that she can decrypt it on her own eventually and Olivia says that it's best if she just focus. Astrid then gets an idea and realizes that each symbol has multiple meetings and she'll need more computational power to run multiple decryption programs.
While Olivia consults with Anil, Walter checks with his son and warns that the cube is too dangerous. He admits that it is possible to destroy the shipping corridor but warns that it is extremely dangerous. He uses a sheet strung in a square to demonstrate how the wormhole works. Walter speculates that if they collapse their end then everything that goes down the wormhole would weigh it down, creating a black hole. To do so, they would neat anti-matter, and Peter suggests they use the anti-matter baton explosives from Etta's apartment. When Olivia warns that it will be dangerous given how many Observers and loyalists will be present, Walter says that they can use the cube to anticipate the signal and get there first. When he points out that Anil couldn’t get the Observer to reveal how the cube works, Peter says that he has his own ways.
As Peter leaves, Olivia suggests they take it slowly but Peter points out that they have nothing so far and he wants to do something tangible. As Walter secretly listens in, Olivia admits that she's afraid and Peter assures her that they can do it and dedicate their lives to fulfilling Etta's dream of saving the world. His wife says that she wants that too and Peter says that they can do it together.
Anil takes Peter to the warehouse where they're negating the Observer's internal tech that keeps him awake and anchored in the current time and place. They wake the Observer up and Anil warns him that Peter is meaner than he is and then leaves them alone. Peter takes out the cube and informs the Observer that he's going to help him put it together. When the Observer says that he has no use for it, Peter says that he plans to turn it on. The Observer says that Peter will get no information from him, reads his mind, and states that Peter believes he can succeed. Peter invites him to read his mind and tells him that it doesn't matter what he learns because the Observer is never leaving and he knows the Observer is feeling fear. The Observer simply stares at him and says he doesn't know what he doesn't know, but Peter says that they'll see.
The Observers supervise the addition of the new parts to the air-conversion device.
Peter begins his interrogation, hooking up a monitor to watch the Observer's eye reactions, and asks what the proper order is to assemble the cube. The Observer remains silent and Peter holds two parts up. He successfully puts two pieces into the cube and then a third, and the Observer says his skill is of no consequence. He dismisses Peter as inconsequential, like an ant colony, and tells Peter that he is oblivious to his limitations. Peter continues assembling parts and figures that the Observer is worried.
As Olivia looks at Etta's necklace, Walter come in with a tape and explains that it was in his desk. It's a video of one of Etta's birthdays and says that it's important that Olivia and Peter see it. Olivia warns him that she's holding on by a thread and Walter says that he overheard her saying earlier that she's afraid. He knows that he knows she's afraid of losing Peter again and that's why it's important to watch the tape so that they remember what they had and hold onto it. Walter tells Olivia that she and Peter must hold onto the pain because it's Etta's legacy to them and they can't escape it by breaking the universe or healing their hearts away. Olivia says that she can't do it as Astrid comes in and says that they decrypted the book and the next shipment is that afternoon. Olivia leaves and Walter puts the tape down on the table.
Peter warns the Observer that if he attaches the next piece then they’ll be obliterated. He asks which way is the right way of four possibilities and the Observer says that he has no fear of death. Peter says that the Observe was once human and will try to stay alive no matter what. He points out that the Observer's body will react on a subconscious level, his pupils dilating when he feels safe. Peter watches the Observer's eye dilations and puts in the part successfully. Olivia calls to give him the address where the shipping lane is opening in 30 minutes and asks if he'll be ready. Peter installs another part and hesitates, and then successfully turns the device on. He tells Olivia that he's on his way.
The team takes the cube to the address and activates it, and Peter sets up with a grenade launcher at a warehouse. When they activate their device, the portal opens and the Observers setting up their device realize that something is wrong. An Observer teleports into the warehouse and attacks Peter and Olivia. Peter smiles in satisfaction as the Observer prepares to kill him, just as Olivia kills the intruder with a gun that shoots bullets they can't anticipate. Peter then fires the anti-matter baton into the wormhole. The cargo containers are sucked through and the portal disappears.
Peter and Olivia get to the van where Alin is waiting. However, he realizes something is wrong and they pull over. They look down the alleyway and see the cargo containers coming through a new portal. They call Walter, who insists that it should have worked. Peter goes down the alleyway, telling Olivia that he's going to get answers. As Olivia starts to go after him, she notices posters with Etta's face on them, promoting resistance to the Observers.
Peter goes back to the resistance warehouse and tells Niemann to leave. He then takes a plastic sheet and smothers the Observer for several seconds. He finally relents and asks why it didn't work, and the Observer tells him again that he doesn't know what he doesn't know. The Observer echoes Alin's words taken from Peter's mind about taking revenge. He explains that focused on a fly and his reaction caused his pupils pupil to dilate. Peter believed what he wanted to believe because his emotions fooled his perceptions. Peter doesn't believe it but the Observer says that he put together the device using his experience as an engineer. He just interpreted the Observer's dilation to justify his decisions. The Observer reads Peter's mind and says that Etta is irrelevant, but Peter says that she will be remembered and no one will remember the Observer once Peter kills him. Peter says that he'd be ten times what he is if he had the Observer's tech in his head and clubs the prisoner unconscious.
Back at the lab, Olivia finally picks up the tape and puts it in the Betamax machine.
Peter takes out some surgical tools and tells the Observer that emotions are real and his feelings for Etta are very real. He then starts cutting open the back of the Observer's skull. The Observer twitches in agony and Peter tells him that it's the pain a father feels when he loses his child.
Olivia watches Etta's birthday party and cries.
Peter goes to a mirror and carefully inserts the Observer technology into his own brain. Olivia calls and asks him to come home, saying that Etta wanted them to be together and survive. She tells Peter that she loves him and Peter says that he loves her too before moaning in pain. He hangs up the phone and stares at himself.