Fringe/Recap/S04/E02

Season 4, Episode 2:

One Night in October
In the alternate universe, efforts to apprehend a dangerous serial killer named John McClellan have come to naught, so Fauxlivia proposes a novel use of the bridge between universes: she wants to recruit the prime universe version of McClellan, who is a professor of forensic psychology, to help apprehend his counterpart. The prime universe Fringe Division agrees, and Olivia recruits prime!McClennan for some top secret profiling work for the FBI. He is sedated, brought to the alternate universe and revived in the company of Fauxlivia, who pretends to be Olivia. McClennan profiles the serial killer, but starts to realise something is amiss when he finds a picture of his own father. He tries to run away, and discovers the nature of the alternate universes.

Once he knows that the killer he's profiling is his alternate self, McClennan admits that he too has the urge to kill, but is able to surpress them because of his experiences with a woman named Marjorie during his youth; he became a forensic psychologist in order to better understand and help people like himself. Later, he waits until the Fringe agents are distracted and slips away, determined to hunt down his alternate self. Based on McClennan's profile of the killer, the Fringe team also try to track him down. McClennan finds his double first, and the discover that the alternate!McClennan never met Margorie and therefore never learned how to surpress his urges. Alternate!McClennan hooks prime!McClennan up to his machine that allows him to "download" other people's happy memories, and takes prime!McClennan's memories of Margorie. By the time the Fringe agents track them down, the memories have taken hold and alternate!McClennan, feeling remorse for the first time in his life, commits suicide.

Back in the prime universe, McClennan wakes up with no memories of his time in the alternate universe -- and no memories of Marjorie. Olivia worries that this will cause McClennan to start behaving like his counterpart, but he appears to be the same, and Broyles posits his own personal belief that people leave a mark on one another's souls that can't be erased, no matter what happens to their memories.

In the meantime, Walter remains in the prime universe. He continues to believe he is going insane as he hears a voice -- Peter's voice -- begging for his help.