In his kitchen lab at home, Walter is preparing a chemical concoction and prepares to inject it into himself. As he drops his pants, Peter comes down and asks what is going on. His father explains that he's trying to make himself smarter, to match Walternate. Peter notes that Bell removed the pieces of Walter's brain, at Walter's request, and he worries that his father might hurt himself. Walter insists that he's fine and goes to get some pizza.
At a senior care facility in Boston, Massachusetts, the staff notices Mr. Roscoe Joyce walking down the hallway. A nurse warns the security officer that Mr. Joyce is a sleepwalker. They see a teenage boy open the door and greet Joyce, and they go to find Joyce. He says that the boy was Bobby. As they return him to his room, the nurse explains that Bobby Joyce died in 1985, and was Mr. Joyce's son. They ask Mr. Joyce what the boy said, but get no response.
On the lawn, Bobby approaches the Observer, September, who asks if Bobby told his father what he needed to. When Bobby says he did, the Observer says that now he takes him back home.
A messenger arrives at Olivia's apartment and gives her a package. When she opens it, she discovers that there is a book inside, If You Meet the Buddha on the Road, Kill Him! Inside is card identifying it as a present from Peter. Before she can look at it further, she gets a call summoning her to the hospital. She goes there and meets Peter, Broyles, and Walter. Broyles explains that Bobby Joyce appeared at the hospital and talked to his father, and then met with the Observer outside. The nurse takes them to Mr. Joyce, and Walter recognizes him. He runs to Joyce's side and eagerly shakes his hand, and insists that he's a tremendous fan. Peter explains that Joyce was the keyboardist for a 1970s band, Violet Sedan Chair. Olivia talks to Joyce, who says he doesn't remember what his son said, but that it was a miracle seeing him again. The staff takes Joyce away for his physical therapy, and Walter suggests that the Observer brought Bobby from the past. He wants to take Joyce back to the past, and notes that every time the Observer shows up, it has something to do with Peter... and something bad happens. Walter walks away while Broyles makes the arrangements to have Joyce released.
As Peter picks up Joyce's medication, he notices Olivia watching an elderly couple talking and laughing together. She gives him the book back and says that based on the date, he must have gotten it for Bolivia. Olivia tells him that it's okay. As Joyce goes with them, he eyes them suspiciously.
Burglars are robbing a jewelry store when September comes in and subdues them one at a time. He catches their bullets as they shoot, knocks them out, and then calls the police to report a robbery. As he walks out, one of the captive staff members, Victoria Dimorio, notice him. She's suffering from asthma, and he removes her medication and gives it to her. She thanks him, and he walks away, taking the inhaler with him.
Walter has a piano brought to his lab and prepares Joyce for hypnotherapy so they can recover his memories. He believes that Joyce playing the piano will help him recollect the conversation, and admits that he'll enjoy Joyce playing. Walter then starts to hypnotize Joyce. In the office, Olivia is checking on the sighting of the Observer at the jewelry store. Peter comes in and explains that Bolivia asked him what his favorite book was. He admits that he has a tendency to keep people at bay, and believes that Olivia is the same, and that he wanted to share it with the Olivia Dunham he has spent two years with. Olivia tells him that everything feels different, and the book reminds her of all the things she missed.
Walter succeeds in hypnotizing Joyce, and has him sit at the piano. Joyce begins playing and Walter has him think back to the night Bobby came. The musician talks about how he asked his son if he was real, and Bobby took his hand to assure his father that he existed. Olivia's phone rings, interrupting Joyce before he can say what Bobby said. As she takes the call, Joyce says that he can't remember what his son said. Olivia tells them that the Observer has been spotted in Brookline. She leaves with Peter, and Joyce says that Bobby was wearing the shirt that his mother gave him, and told Joyce that he would meet Walter Bishop and was supposed to help him. However, Joyce doesn't know how he's supposed to help Walter.
In Cambridge, the Observer meets with one of his fellow Observers, December, and says he has set everything in motion. December says that he doesn't believe Walter will do what they need him to do, but September insists that Walter has changed. They agree to see what happens, and September then steals a truck and drives away.
Joyce is playing the piano and asks if Walter knows what he's supposed to do for him. Walter insists that Joyce is helping just by playing the piano, and then asks him why Violet Sedan Chair broke up. Joyce explains that they had creative differences and decided to take a break, and Joyce soon stopped playing entirely.
Peter and Olivia talks to Victoria, who says that September knew exactly what she needed, but didn't react at all.
Walter is working on the serum to repair the missing sections of his brain, and explains to Joyce that he needs his restored intellect to deal with the challenges facing them. Joyce has no idea how he can help Walter, since the only science he knows is how to make a strawberry milk shake. Walter tells him that strawberry milk shakes are his favorite drink as well, and has Astrid go to the store to get the ingredients. Walter mixes the serum with milk to combine the bonding agents and disguise the taste, , and prepares to take a drink. September enters the lab and says they need to speak.
Walter and September walk in the park, and Walter reminds him that they had a deal. He asks how he can stop Peter from dying, but September warns that he can see many futures, but he doesn't know which one will come to pass. Each action has consequences, foreseen and unforeseen. September explains that later that summer, after he rescued Peter, Peter caught a firefly. Because he did, a young girl three miles away did not, and she kept looking. Her father went to look for her in his truck and killed a pedestrian in an accident at Harvard Yard. September warns that they have both upset the balance of events, and now he needs Walter's help. He tells Walter to give Peter the keys and save the girl, and then tells Walter to answer his phone. Walter's phone rings and he answers it, and Peter tells him about the jewelry store incident. When Walter turns around, September has disappeared. Walter asks to speak to Victoria and says it's important. Peter says that the police are taking her downtown, and Walter asks his son to bring her to him when she's done.
In the lab, Walter finds Joyce eating strawberry ice cream. He explains that 25 years old, Bobby called him on the phone years ago when he was on tour. Bobby told him about a dream of a bald man in a dark suit taking him to see Joyce when he was 25 years older. Walter concludes that September brought Bobby into the future, and his dream was what actually happened. Joyce notes that it was the last conversation he had with his son before Bobby died, and the band was playing at Harvard Yard. Bobby was on his way there and was hit by a truck driver. When Bobby died, Joyce broke up the band because nothing mattered to him anymore. Walter goes to the office to see Astrid, who tells him that Peter is on his way with Victoria. Walter tells Astrid that he now realizes that he set both universes off balance by saving Peter, and that his actions cost Joyce his son. However, if he helps September now, it will cost him Peter.
As Olivia and Peter drive to the lab with Victoria, Walter calls and asks Peter to ask Victoria where she was 25 years ago. Peter tells his father that Victoria is in the car ahead. Before they can stop it, a truck drives through the intersection and hits the car containing Victoria. Peter and Olivia get out and see September as he gets out of the truck. Olivia goes after him while Peter checks on Victoria. She's having an asthma attack, and September took her inhaler at the jewelry store. Walter arrives and Peter says, "Gimme the keys and save the girl." Walter remembers September saying the same thing, and realizes that somehow the Observer is course correcting, setting off a chain reaction leading from Joyce and Bobby, all the way to the current moment. Walter believes that Peter will die if he leaves, but Peter insists that Victoria will die if he doesn't get the key. Walter hesitates and then gives him the keys, and Peter goes to the car.
Olivia follows September through the streets. He ducks down an alleyway on State Street, and Peter spots him at a hostel down the way. As Peter crosses the street in pursuit, he's narrowly avoids being hit by traffic.
Walter creates an improvised inhaler using a plastic bottle to push air into her lungs.
Peter goes to the roof of the hostel and finds the Observer waiting for him. He asks September what it all means, and what is going to happen to him. September says that it must be difficult being a father, and then shoots Peter with a pulse gun. Olivia arrives and spots him, but he climbs to a nearby roof and then appears on the next building over. Before Olivia can approach him, she hears Peter moving behind her and turns to discover that he's okay. When she turns back, September has disappeared.
Walter has saved Victoria and briefs the EMTs on how to treat her. Olivia calls to tell him that September has disappeared, but Peter is fine. Walter wonders why the Observer went to all the effort if Peter is still alive.
Walter and Astrid take Joyce back to the hospital. He thanks them and invites Walter to visit sometime, and bring a strawberry milk shake. Much to Walter's surprise, Joyce hugs him and then explains that he had forgotten his son, but now he remembers. He tells Walter is that no one is supposed to have a second chance like that.
Peter and Olivia return to the lab, and wonder what the Observer had planned. Peter takes out the bottle of milk that Walter had put there earlier. Olivia picks up Peter's book and Peter starts to talk about the book and that it is his favorite because it says to look within yourself for answers. He drinks the milk to take some aspirin for his pain, and then has a seizure. Olivia calls Walter, who realizes what has happened and directs Olivia in administering magnesium sulfate. However, she doesn't find it in his bag. Walter tries to remember where it is and tells her that it's in the refrigerator next to the mayonnaise. She injects it into Peter's right leg and then tells Walter that he's stabilizing.
At home that night, Walter makes rosemary chicken soup for Peter, who is recovering on the sofa. He thanks Peter for proving that the serum was flawed. Peter only lived because he was young and healthy, but it would have killed Walter. Walter figures that September set the whole thing up so Peter would take a knock on his head, so he'd take the aspirin with milk, and drink the serum.
Outside, September and December watch. September admits that he thought the experiment would change, but December points out that Walter has changed and was willing to let Peter die. They believe that now Walter will be willing to do it again.