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This is Season 4, Episode 02, which comes after the last
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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.
Synopsis provided by the Fandom Wiki, Fringepedia, and Wikipedia.
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.
Timestamp: 40:38
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'.
Timestamp: 29:56
Timestamp: 13:06
FRINGE DIVISION CASE FILE #402-93011
FRINGE EVENT: Forced extraction of human memories
CLASSIFIED CASE CONNECTION(S): Serial killer, Over There universe
AGENTS ASSIGNED: Dunham, Lee, Bergmann (OH)
SUPERVISING AGENT: Broyles, Colonel Broyles
FILE ARCHIVED BY: Marvin Sommershof
WITNESSES/CONTACTS: John Louis McClennan (suspect; deceased; OT), Megan Miller (OT)
LOCATION: Norristown, PA (OT); Weir Farm, PA (OT)
VICTIM(S): Jeremy Roman (deceased; OT), Noreen Miller (OT), John Louis McClennan (OT)
CASE REPORT SUMMARY:
Timestamp: 12:12
Timestamp: 27:05
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.