Complete story — includes the ending
Alternate Astrid discreetly visits her counterpart to talk about her recently deceased father. Fauxlivia is sent to escort her back. Peter and Olivia track a killer targeting people who will die a painful death to save them from suffering, using a toxin from the future. With help from Alternate Astrid, they identify their man as a TSA agent who cleared all his victims prior. He gained knowledge of their future by solving certain equations when he was a mathematics professor at MIT. Having survived a car accident that took his father and twin brother, he overheard his mother saying that God had taken the wrong son, which motivated him to act as a savior. When the Fringe team is about to arrest him, he fires a gunshot towards Olivia, purposely missing her, upon which she returns fire and kills him. The parallel versions of Astrid and Olivia return to their universe after warm goodbyes. Two Observers, including December, learn the killer found the toxin he used on Reiden Lake; it had been lost by September when he had unsuccessfully tried to save Peter. December learns that Peter has reappeared in this timeline.
Sources and content license
Source and revision history · Adapted from Wikipedia contributors, Fringe season 4. HTML formatting and reference markers removed. · Content license