As David Robert Jones uses a captive Nina in an effort to activate Olivia's Cortexiphan-enhanced abilities, the dying Observer visits Peter and Walter at their lab. Peter must merge his mind with September's in order to learn his secrets.
This episode's glyphs spell:
The Observer is featured in this episode and Peter goes into his mind.
A newspaper ad in 'A Better Human Being' that says "Your Travel Adventure - Sun Passes" hints at where Peter goes with the Observer, and what Peter learns there, in 'The End of All Things'.
A door marked "Burn Unit" in 'The End of All Things' hints at the villain's burned face and burned marks on his victims' necks in 'A Short Story About Love'.
Walter watched a video of the aftermath of Olivia's explosive childhood incident in "Bad Dreams". (Image 1)
In "The Boy Must Live" September revealed that he was experimented on by other Observers and was biologically reverted to a purely human state as punishment for his interventions.
In "An Enemy of Fate" the Fringe team successfully changed the future so that Observers would never be created.