Late at night, Peter catches Walter about to inject himself with some green goo, trying to counteract the damage done when William Bell removed those pieces of his brain years ago. He needs to think like Walternate now, so he can figure out what he's trying to do with the device and keep Peter safe. Peter reminds him that Walter asked Bell to do that, because he was afraid of what he was becoming. Walter jabs himself anyway.
In a Boston nursing home, Roscoe Joyce sleepwalks - and meets his son, Bobby, who died in 1985. Later, outside, the young man approaches the Observer, who asks, "Did you tell him?" Bobby nods.
The next morning, Olivia gets a package, a book titled "If You Meet the Buddha on the Road, Kill Him!" Inside is a note from Peter: "Olivia . . . Because you asked." Then she's called to the nursing home, where Broyles shows the team a security photo of Bobby with the Observer. The nurse says that Mr. Joyce doesn't remember what Bobby said. Walter knows Roscoe: He's the eccentric keyboardist of Violet Sedan Chair, Walter's favorite old band.
Walter says that the Observer can travel in time; maybe he can take people with him. While waiting to transport Roscoe to the lab so Walter can help jog his memory, Olivia gives Peter the book. She thinks it wasn't for her.
Elsewhere, the Observer foils a jewelry store robbery. The salesclerk is having an asthma attack. He gets her inhaler, gives her a hit, and leaves, taking the inhaler.
That piano in the lab is for Roscoe's hypnotherapy. Playing it might help him remember, plus Walter would enjoy it. Meanwhile, Peter tells Olivia that Bolivia asked what his favorite book was. He says he usually keeps people at arm's length - something they have in common. The book wasn't for "her"; it was for Olivia. He wanted to share it with her. Olivia says that everything's different, even Peter opening up. The book just reminds her of all she's missed. Astrid interrupts, calling them into the lab.
The hypnotized Roscoe is playing piano and is about to recall what Bobby said. Then Olivia's phone rings, shattering the moment. The Observer has been spotted. Peter and Olivia take off. Then Roscoe remembers: Bobby said he would meet Walter Bishop, and Roscoe is supposed to help him. But how?
The Observer meets another Observer, who doesn't think Dr. Bishop will do it. The Observer disagrees; he believes Walter has changed. Then the Observer steals a pickup truck. At the jewelry store, the clerk tells Olivia and Peter that the Observer saved her life.
Roscoe plays piano while Walter pours his brain-boosting serum into a bottle with some milk and puts it in the fridge. They bond over their mutual love of strawberry milkshakes. Walter is about to drink his concoction when the Observer arrives for a chat.
Outside, Walter fretfully asks about Peter and the device. The Observer sees many possible futures but can't tell which will occur. Every action causes consequences, both obvious and unforeseen. For instance, when the Observer rescued the Bishops from the lake, he couldn't have known that Peter would later catch a firefly, setting off a chain reaction ending with a truck skidding through the rainy intersection at Harvard Yard and killing a pedestrian. They've both upset the balance, and now the Observer needs Walter's help. "When the time comes, give him the keys and save the girl," he says mysteriously.
Peter phones Walter about the girl at the robbery. The girl?! Walter needs to talk to her! OK, after the cops take her statement.
Roscoe remembers Bobby phoning him, years earlier, about a strange dream: A bald man in a dark suit took him to see Roscoe as an old man, in a nursing home. Walter realizes it wasn't a dream. That night, Roscoe continues, the band had a gig in the Harvard Yard. It was raining, hard. Walter knows what's next: A truck skidded through the light, killing Bobby.
Distressed, Walter tells Astrid that Roscoe lost his son because he wasn't willing to lose Peter. He phones Peter, who's with Olivia, following the police car carrying the salesclerk. They're almost to the lab. It's raining. Suddenly, a pickup truck - the one the Observer stole - rams the police car! Olivia chases the Observer as Peter tells Walter what happened. The salesclerk is having an asthma attack, and her inhaler is gone.
Walter drives to the scene with Astrid. Peter says, "Give me the keys and save the girl" . . . and Walter unravels. If he does, Peter will die! Peter insists. Walter hands over the keys and saves the girl. Later, he and Astrid take Roscoe home. Roscoe is grateful that he saw his son: Nobody is supposed to have a second chance like that.
The Observer hits Peter with a concussive blast from his gun, then disappears. Back at the lab, Peter tells Olivia that the book is his favorite because it talks about not depending on others for answers; you can only find them inside yourself. He takes aspirin for his headache, unknowingly downing it with Walter's concoction - and collapses! But Olivia phones Walter and gets the antidote.
Later, at the Bishops', Walter tells Peter that the flawed serum would've killed him. Now he thinks that the Observer's plan was to save his life by giving Peter a headache, so he'd use the milk instead.
Or not. Outside, the Observer discusses his experiment with his counterpart, who admits he was wrong: Walter has changed. He was willing to let his son die. Yes, says the Observer. "And now we know, when the time comes, he will be willing to do it again."