In the episode, Olivia is informed by Broyles that the Fringe Division of the alternate universe has requested their help to locate a serial killer named John McClennan. With their own universe's John, a university professor, they try to locate the other John with his knowledge of himself while also trying to prevent him from being aware of the alternate universe itself as they cross over.
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This episode's glyphs spell:
A limbus is a distinctive border or edge. Professor McClennan's fate was balanced on a knife's edge; the presence or absence of one single interaction determined whether he'd be a professor or a serial killer.
The Observer can be seen in the hospital behind Olivia and Broyles.
A rail tanker in 'Neither Here Nor There' has "Brain Freeze" spray painted on it, foreshadowing the memory extraction process used by the killer in 'One Night in October'.
Professor McClennan's professional library in 'One Night In October' contains the book Killer Mindscapes by S. Pores, foreshadowing the fungi that can form mental connections with people from 'Alone in the World'.
The case in 'One Night in October' centers around two versions of the same man, each from a different universe, where one is a serial killer and the other is a professor of forensic psychology specializing in serial killers.
This happened differently in the timeline without Peter.