Olivia is asleep at Etta's apartment as Peter finds a photo of his dead daughter. He holds it up, comparing it to his wife, and then searches further. He locates a secret compartment holding explosives behind a painting. As Peter packs them away, Olivia wakes up and realizes that she still thinks that Etta dying is a nightmare. Crying, Olivia wonders why they would get Etta back just to lose her again and Peter holds her. As she goes into the bathroom, Olivia sees the family photos that Etta kept and starts packing them away.
On the streets of Manhattan, bursts of static electricity start to form and the Observers bring in a squad of loyalists to secure the area. They place a cubical device in the middle of the street and then record everything in their journals. A portal opens and three large platforms covered in white cloth float through. The portal shuts down, charring the street around it, and the Observers confirm that the components on the platform came through safely.
As Olivia packs away Etta's things, Walter comes out and asks to keep a bottle of the girl's perfume. He explains that due to his memory problems, he wants a distinctive odor to trigger memories of his granddaughter. As they talk, Astrid calls from Harvard and tells them that the next tape is on a table next to a propane tank and a gallon of ethanol. Walter left them there and it will take her extra time to use the laser to cut past them. Etta's partner in the underground, Alin, calls on Etta's phone and asks for a meeting.
Peter, Olivia, and Walter go to Manhattan and Alin arrives. He offers his condolences and tells them that he hopes one day they'll understand how much Etta meant to the resistance. Alin then shows them the charred spot on the street and explains that the Observers use wormholes to bring parts for their air-degradation system from their future. If they finish the machine, one of several scattered across the world, then the Observers will successfully convert the atmosphere to their standards and eventually wipe out humanity. The resistance wants to destroy the next ship and Alin shows them an Observer's journal. It's in the Observer's language and the resistance needs someone to translate it. When Olivia wonders how they got it, Alin explains that they captured an Observer at the site and are holding him at a secure warehouse. Peter notices a cube-shaped pattern inside the burn and realizes that it generated the wormhole. Alin confirms that the Observer had tech on him and Peter suggests that they use the device and destroy the transit corridor. Alin and Walter both advise against it, warning that they don't understand the technology, but Peter insists.
As Walter prepares to get some samples, Olivia goes to the van. Anil tells Peter privately that if he's seeking revenge then he should prepare two graves. Peter assures him that he's not worried about dying and asks for the cube. Alin reluctantly calls Niemann, the resistance member guarding the Observer, and has him bring the cube. Once Niemann brings it, Peter takes it back to Harvard and tries to work out how to assemble the pieces. Astrid and Olivia try to decipher the journal. When Astrid realizes that Olivia is distracted, she suggests that she can do it on her own but Olivia says that she wants to keep working. They realize that in the Observer's language, each symbol represents multiple meanings and Astrid says that they'll need more computational power to run multiple decryption programs.
While Olivia talks to Anil about getting more computers, Walter tells Peter that the cube is too dangerous. However, he admits that they may be able to create a black hole in the future end of the transit corridor by creating a sufficient explosion within the wormhole. They'll need anti-matter and Peter points out that they have the anti-matter explosives that Etta cached. Olivia warns them that the Observers will secure the transit location but Walter says that if they can get the cube working, they can create a portal wherever they wish and get there first. When they wonder how they will get the cube to work, Peter says that he has some methods that Anil didn't try.
As Peter leaves with Anil, Olivia suggests that he take it slowly. However, a desperate and angry Peter wants something tangible that they can call a triumph and make Etta's sacrifice means something. As Walter quietly listens in, Olivia tells Peter that she's afraid but he assures her that they can save the world and fulfill their daughter's dream. When Olivia says that she wants that, Peter tells her that they can do it together.
At the resistance base, Anil explains that they're controlling the Observer's internal body chemistry to restrain him and negate the implanted tech that keeps him alive. They wake the Observer up and Anil leaves Peter alone with their prisoner. Peter shows the Observer the cube and tells the prisoner that he's going to help Peter put it together. The Observer, unimpressed, says that Peter has no use for it and Peter says that he plans to turn it on. The prisoner reads Peter's mind and points out that Peter believes that he can succeed. Undeterred, Peter tells the Observer that no matter what he sees in Peter's mind won't matter because the prisoner isn't leaving there alive. Looking into the Observer's eyes, Peter says that he knows that he is feeling fear. The Observer simply stares calmly back and tells Peter that he doesn't even know what he doesn't know, and Peter says that they'll see.
The Observers supervise the installation of the new components into their Central Park air-degradation machine.
Peter hooks up a monitor to scrutinize the Observer's eyes and then starts putting the cube together. The Observer refuses to say anything when Peter holds up two parts, and Peter then inserts them correctly into the cube. As he works, the Observer says that his skill is of no consequence and compares Peter and humanity to an ant colony. He has no interest in ants and tells Peter that he has no idea what his limitations are. Peter keeps working and figures that the Observer is worried.
At Harvard, Walter gives Olivia a tape and says that she has to see it. He had it in his desk and it's from one of Etta's birthdays. Walter wants Olivia and Peter to both see it but Olivia warns her father-in-law that she's barely holding on. He tells her that her overheard her say that she's afraid and that knows she's afraid of losing peter again. They have to watch the tape so that the couple can remember what they had and hold onto it. Olivia still isn't convinced but Walter tells her that the pain is part of Etta's legacy to them and they can't escape or block it. Olivia refuses as Astrid comes in and tells them that she decrypted the journal. According to the Observer's records, the next shipment is due that afternoon. As Olivia and Astrid leave, Walter puts the tape on the table.
Peter continues working and warns the Observer that if he puts the next part in wrong then they'll both be obliterated. The Observer says that he has no fear of death, but Peter says that he's been monitoring his pupil dilation. The Observers came from human stock and an instinctive desire to survive. When the Observer feels safe, his pupils subconsciously dilate. Peter tries positions for the piece until he sees the Observer's eyes dilate and then inserts the piece successfully. Olivia calls her husband to tell them the address where the transit corridor will appear. She asks if Peter is ready and he puts in the last piece, successfully turning on the device. He then tells Olivia that he's on his way.
The team goes to the address and Peter and Olivia set up in a building overlooking the street. They activate the device and open a portal near their location, and the Observers at the original site realize that something is wrong. One of them teleports into the building and attacks Peter and Olivia, knocking them down. The Observer Supervisor prepares to crush in Peter's skull but Olivia shoots him with an accelerant gun. Unable to catch the bullet, the Supervisor dies and Peter fires an anti-matter baton into the wormhole. The wormhole collapses, sucking in the cargo containers, and the portal disappears.
Peter and Olivia join Alin at the van but as they drive down the alleyway, Peter sees a new portal open up behind them and more components come through. They call Walter, who insists that his plans should have worked. Peter leaves, telling Olivia that he's going to get answers. As Olivia goes after him, she notices posters of Etta up, all supporting resistance, and she realizes what her daughter meant to the world.
Peter goes back to the resistance base and tells Niemann to leave. He then smothers the Observer for several seconds and then asks why the cube didn't work. The Observer echoes his earlier words, telling Peter that he doesn't even know what he doesn't know. He then reads Peter's mind and recites what Alin said about digging two graves. The Observer explains that he focused on a fly on the window when Peter was assembling the cube, and his eyes dilated in response to its movements. Peter convinced himself that he could read the Observer's subconscious tells to justify his own actions and what his experience as an engineer led him to choose. The Observer reads Peter's mind again and realizes that Peter blames them for his death, and tells him that Etta is irrelevant. Peter tells him that while Etta will be remembered, no one will remember a dead Observer. He angrily says that he'd be ten times what he was if he had the Observers' technology in his head and then clubs the prisoner unconscious.
Back at the lab, Olivia watches the tape of Olivia's birthday and the three of them together as a family.
Peter straps the Observer down and begins cutting out his technology from the back of his skull. As the Observer writhes in agony, Peter tells him that he's feeling the same kind of pain that a parent suffers when they lose a child.
As she watches the tape, Olivia breaks into tears.
Peter goes to the mirror and cuts open a hole in the back of his skull. Olivia calls, interrupting him, and asks him to come home. She says that Etta would want them to be together and survive. Peter places the Observer device at the back of his neck and it leaps into the hole that he's cut and it crawls into his skull. Moaning in pain, Peter tells Olivia that he loves her and hangs up the phone, staring at himself in the mirror.