Season synopsis
Contains spoilers for this season and earlier seasons.
In the altered timeline created after Peter disappeared, neither universe remembers him, and the bridge between them supports a fragile truce. Olivia, Walter, Astrid and Broyles continue investigating fringe events while confronting David Robert Jones, who has returned and is assembling a plan that threatens both worlds. Peter unexpectedly reappears after emerging from the lake where he drowned as a child in the altered history. He insists that he belongs with the team, but his presence feels impossible to people whose memories contain no shared history with him.
As Peter investigates his return, he searches for a way back to the life he remembers. Olivia begins to experience memories and emotions from the original timeline, including her relationship with Peter, even as those memories conflict with the life she lived in this version. September eventually confirms that Peter is already home, and Olivia embraces the returning memories. Jones exploits the two universes’ connection to advance his plan. Walter and Peter work to stop him while the team’s trust is tested by Jones’s agents and by the differences between the two timelines.
The season’s nineteenth episode, “Letters of Transit,” jumps ahead to 2036, showing a future in which the Observers rule and Walter is found preserved in amber by a human resistance group. The finale returns to the present: Jones is electrocuted, but Bell’s plan to collapse both universes continues. Walter shoots Olivia to cut off the power her abilities are supplying to it. Olivia recovers, and Peter discovers that she is pregnant with their child. September then appears with a warning that the Observers are coming. The invasion itself belongs to the future revealed in “Letters of Transit” and becomes the setting of season five.