Fringe/Recap/S03 E10

Season 3, Episode 10:

The Firefly
Roscoe Joyce, the elderly former-keyboardist of Violet Sedan Chair, is visited in his nursing home one night by his son, Bobby, who died more than twenty five years earlier. When the Fringe team investigates, they find evidence that September the Observer was involved with Bobby's visit, and suspect that September had actually brought Bobby forward in time from 1985 and then returned him. Because Roscoe is having trouble remembering what Bobby actually said, the team bring him back to Walter's lab to use some hypnotic suggestion to help him remember, though Walter has an ulterior motive: he is a huge fan of Roscoe and Violet Sedan Chair's music, and in the process of helping Roscoe remember, he gets to hear Roscoe play a bit of music. The two talk and become friends, and Walter shows off something he's been working on, a milk-based serum to help him regrow the parts of his brain that William Bell cut out. Roscoe remembers what Bobby told him: it was a message for Walter.

Meanwhile, the Observer goes about a very complicated scheme. He intervenes in a bank robbery, easily disabling the robbers and helping a young woman who needs her inhaler, then steals it when he leaves. After this, he meets with Walter and attempts to convey the consequences of travelling between universes: a few months after Peter came to the prime universe, he caught a firefly one evening, meaning that a young girl who was supposed to catch that same firefly stayed out later, so her father drove out to find her in a bad storm and accidentally killed a pedestrian. Walter is called back to the lab, but the Observer gives him a message first: "give him the keys and save the girl." When Walter returns to the lab, Roscoe tells him about how Bobby died as a result of a car accident, and Walter realises that Bobby was the pedestrian that the Observer told him about, who was killed as an indirect consequence of Walter's actions.

Olivia and Peter bring in the woman who was saved by the Observer. En route, the Observer crashes a car into the FBI convoy, causing the woman to have another asthma attack but without her inhaler. Olivia chases the Observer while Peter stays to help the woman. Walter, having heard the entire incident over the phone, drives out the the site of the accident. Peter wants to take Walter's car to pursue the Observer, and says to Walter "give me the keys and save the girl." Walter realises what the Observer has done: he's set up a situation in which Walter will have to let Peter go for the greater good. Despite his misgivings, Walter gives Peter the keys and he's able to catch up to the Observer on a rooftop. The Observer comments on the difficulty of being a father before shooting Peter with the stun gun. Meanwhile, Walter creates a makeshift inhaler and saves the young woman's life.

Back at the lab, Peter is having some asprin to help with the headache he got from being shot with the stun gun. Thinking it's milk, he swallows it down with Walter's brain serum and starts to have a seizure. Walter helps Olivia to save Peter's life, and in doing so he realises the point of the Observer's schemes: had Peter not taken the keys, he wouldn't have been shot by the Observer and wouldn't have taken the brain serum, which was flawed and would've killed Walter if he'd taken it. Because Peter is young, he wasn't killed instantly and was able to be saved, if Walter had been unable to let Peter go for the greater good, it would've been Walter who drank the brain serum and died. In a scene between the Observer and his superior, December, this theory is confirmed, and they comment that Walter will now be ready to let Peter go when the time comes.