Complete story — includes the ending
Several wives have died shortly after the passing of their respective husbands. Discovering traces of the husbands' DNA on the bodies of the wives, Walter suspects that the killer is using pheromones taken from the husbands as a perfume to get close to the wives. They identify the murderer as a former employee of a perfume company, and are able to save his last intended victim and apprehend him. As he is taken away, he admits he was trying to discover what love was, so that he could reproduce it.
During the investigation, Olivia realizes that she is in love with Peter. After the conclusion of the case, she tells Nina that she will let the memories she is gaining from Peter's original timeline override her own memories, even if this means she will forget the times she spent with Nina. Simultaneous to these events, Peter has left Boston to stay away from Olivia, so as not to further erode her original memories. Walter calls him back, identifying that the Observer September has implanted something in Peter's eye during the events of the previous episode. The small disc reveals an address. Equipment found there enables Peter to bring back September, who explains that the other Observers had hidden the universe from him. In response to Peter's questions about trying to return to his own timeline, September states that Peter is actually home; he was never truly erased from time as his love for his friends and their love for him brought him back. September vanishes, and Peter and Olivia rejoin each other with a kiss.
Sources and content license
Source and revision history · Adapted from Wikipedia contributors, Fringe season 4. HTML formatting and reference markers removed. · Content license