At the Harvard lab, Astrid wakes up when she hears a sound. She goes over and discovers that the radio that they took from the apartment is receiving a signal. Walter is sitting in the dark and he tells her that her hair is beautiful. Astrid asks him if he fixed the radio but Walter simply says the same thing again.
Peter is trying to sleep without success and Olivia finally wakes up and notices. He complains that he's having headaches again and assures her that she doesn't need to get anything. Astrid calls them to the lab and tells them that the radio signal is repeating some kind of pattern. They wonder if it's Walter's mysterious ally Donald and Astrid informs Peter and Olivia that Walter dropped acid. Peter tries to get through to his father, but he's busy imagining his old lab assistant, Carla Warren.
As Walter stares at a small green pixie, Peter tries to get his father to acknowledge what's going on. Walter explains that he took black blotter acid that he had in the storeroom, and that Nina agreed to remove the pieces of his brain that they put back in once he completed the plan. Peter says it isn't a good idea but Walter insists that he doesn't want to lose them and that they're running out of time. Two nights ago he found himself dressed and ready to leave at 3 in the morning. Carla says that "he" knows and Walter echoes her, and Peter realizes that he means the Walter that he used to be. Walter wonders what happens if he doesn't stop himself the next time and wants to produce the rest of the plan before his other self takes over. He then tells Peter that he needs to figure out the tones on the radio and his son leads him away.
Olivia listens to the signal and suggests that they locate the source of the signal. When Peter comes back, he agrees to contact Anil and see if he can help them.
In his room, the green pixie brings Walter a match and he uses it to light a candle. He then goes out into his hallucinatory version of the lab and a red pixie joins the green pixie, and he watches a video tape of Anil showing the others how to track the signal. Walter snaps back to reality as Peter tells him that Anil is going to show them how to use the tech to find the signal. Carla tells Walter that there's something hidden in the lab, while the others tune the device and find a location for the signal in Willington, CT
The next day, Peter and Olivia drive the woods outside of Willington and proceed on foot. As they walk, Peter admits that he's embarrassed that he left her again after promising not to. He says that he doesn't deserve Olivia and is lucky to have her. They kiss and then Olivia notices a camper parked nearby. They approach it and find the remains of a skeleton. It contains Observer tech and they realize it was one of the invaders. Olivia finds two more corpses, an Observer and a loyalist, and evidence that they were killed in a firefight ten to fifteen years ago. There's a skeleton seated in the driver's seat of the camper and Peter figures that it's Donald. It's holding an early prototype Observer gun and they figure he killed the other three and then died of his wounds.
Peter continues tracking the signal and discovers that it's coming from a relay. Meanwhile, Olivia checks the corpse and finds its wallet. She goes to Peter and shows her that the wallet confirms that the dead man was Sam Weiss.
They radio to Astrid and have him ask Walter about Sam Weiss, but Walter doesn't remember anything about Weiss. Olivia figures that Weiss was trying to protect the relay point and tells Astrid that they're recalibrating the tech to find the point of origin for the signal. Once Astrid steps out, Carla gives Walter hints to the hidden cache. She finally tells him that he's looking for his secrets and reminds her that he once insisted he was a god. Walter insists that he's not that man and Nina appears to agree with him. However, Carla says that the real Walter would go to the Observers, show off his knowledge, and receive their adulation. She tells Walter to get ready to leave, but Walter refuses to break his promise to Peter.
Walter goes to his office and finds Carla waiting for him, saying that he's getting close. He approaches the desk she's sitting on and Walter realizes that there's a hidden compartment in the floor. Walter opens it and removes a book, and Nina tells him to put it back. Carla, now a charred corpse, tells him that it's his life's work that she came to burn on the night she died. The green pixie flies over to stop Walter but he shoos her away and decides to take just a peak. When he sees the contents, Walter smiles in satisfaction.
Later, Walter takes a taxi into New York and reads over all of his old design notes while the taxi driver goes to make change. As Carla watches, he finds a reference to Black Umbrella and asks her why it's in his head. A waiter offers him more LSD and Walter takes another dose. When Carla asks if he remembered where how he planned to create a new universe, Walter insists that he isn't that man. He realizes that he's at the Observer Precinct and Carla explains that he tricked Astrid and snuck out of the lab. Walter doesn't believe he would make a decision like that and Carla tells him that he's in denial, which is why she's there. She explains that she represents all of the things that he's trying to keep buried, and Walter explains that he wanted to cry for her so he made himself a different man. Carla warns him that he's clinging to his newer self but he can't hide from what he really is. As Walter prepares to get out, he sees an Observer approaching the car.
Astrid suddenly opens the taxi door and says they're waiting for him. She ushers him out and Walter discovers that he's at a pier. Astrid says that they're meeting Peter and Olivia and renting a boat. They finish making a deal with the owner, Gerald, and go to the boat. Peter tells his father that the signal is coming from an island and Walter sees a green castle on it in the distance, while Peter says that they're going to see if Donald is there.
As they go down the walkway, Walter panics and Peter manages to calm him down. A loyalist patrol boat docks at the pier and hails the group, and Peter claims that they're going out to visit their uncle on the island. When the Loyalists demand their papers, Peter and Olivia open fire and take out the patrol, and the board the rental boat and take off.
The group lands the boat on the island beach and finds a path heading in the direction of the signal. Walter sees Carla again and hesitates, but Astrid pushes him on and assures Peter that Walter is fine. They approach a house and an older man comes out on the porch with a gun. He demands to know who they are and Olivia says that they're there to meet Donald. The man says that there's no Donald there and Peter asks if he recognizes Walter. He doesn't and tells them to leave, but the child Observer comes out. A woman, Carolyn, comes out and calls to her husband, Richard. Olivia explains about the signal and Carolyn tells Richard to put the gun down. Richard says that the person the message was for would know the password.
As Walter stares at the child Observer and tries to remember, a hand picks him up and puts him on a cow. He rides through a factory and meets the green pixie, who leads him to a knight with a key. A frog eats the knight and Walter uses the key to open a tree and take out a black umbrella.
Walter tells Richard the password is "Black Umbrella." Carolyn invites them inside and as Walter goes in, the green pixie applauds him. Inside, Richard tells them that he and Carolyn helped the cause, giving food and shelter to the resistance. The couple acquired a reputation as people who could be trusted and Donald approached them eight months after the invasion began. He said he had to go away and asked them to watch the boy because he was an important part of the plan to defeat the Observers. Donald didn't tell them where he was going and they never saw him again. Before Donald left, he gave them a transmitter and told them to activate it every five days. Carolyn explains that the boy hasn't spoken or aged a day since he came to live with them, and that they named Michael. Richard admits that it was hard to take the chance he would lose their adopted child, but he knows that Michael is intended for something great.
Donald and Carolyn say their goodbyes to the boy and then Michael goes with the team. Back at Harvard, Olivia gets Michael hot cocoa and explains that her daughter loved hot cocoa when she was young. The boy hesitantly takes a sip and then smiles. Olivia asks Michael if he remembers her and he nods. Peter points out that the boy met them in a different timeline and couldn't remember them, but Olivia points out that Observers experience time differently.
In the lab, Walter watches hallucinations of the past when his old self told Carla that he was God. He also sees scenes of Elizabeth complaining about how Walter was never there for her when he was trying to bring Peter back from the other universe. Next he watches memories of himself going through the wormhole and Carla warning him that there has to be a line that he can't cross... and Walter vowing not to let his son die. Once the memories fade, Walter burns his journal. Carla warns him that it's too late now that he remembers everything. Nina tells him that he has to keep fighting now that he has Michael, but Carla tells him that he's too tired and that there's nothing to burn. Walter looks down and realizes that nothing is burning, and his original self appears before him.