This episode is a flashback episode, told as Walter Bishop reveals to Olivia Dunham that his son Peter is really the Peter of the parallel universe. Walter explains the events that occurred in 1985 that led to this and the impact it had on the parallel universe to be at war with the prime one.
This episode's glyphs spell:
This episode is the first and only time we see two different Peters and it tells the story of Walter desperately trying to save them both.
The Observer is featured in this episode.
A police barricade in 'Jacksonville' says "DO NOT CROSS", foreshadowing the conflict between Nina, Carla, and Walter as Nina and Carla try to stop Walter from crossing universes in 'Peter'.
Two Observers in 'Peter' exit a movie theater displaying a poster for "Clue", foreshadowing the title of 'Olivia. In the Lab. With the Revolver' and the confrontation between Olivia and James Heath.
A reference to the fact that Peter Bishop is not from this universe.
Because he knew the universe would start to deteriorate due to his actions in 'Peter' and it would be traced back to him.
Walter made the same mistake, as shown in "Peter".
We learn the full extent of Peter's medical history, and what Walter did to save him, in "Peter".
Everything happening in Fringe can be traced back to Walter stealing Peter as seen in "Peter".
Walter's often not too interested in the personal aspects of cases, but this reminds him of what happened to Peter in "Peter".
They belonged to the Peter that died in "Peter".
Walter was stressed when he hid the pieces of the machine in safety deposit boxes 23 years earlier. This episode takes place in 2008, so 23 years earlier was 1985, the year Peter died in "Peter".
This is because the scar was on the other side in the Redverse before he was stolen in "Peter".
Peter was stolen in "Peter" and was lost from Walternate's point of view, though we know he's alive and well.
Walter showed how far he would go, across universes, in "Peter".
Both Peters have their own coin in "Peter".
Walter kidnapped another Peter in "Peter".
Both Peters in "Peter" had the same favorite coin.
Walter's real son, who died in "Peter", was the one who loved custard.
Just like Peter, who doesn't know that he took the place of the real Blueverse Peter who died in "Peter".
This mirrors the struggle Walter went through in "Peter".
Peter has the same glow about him after was taken from the other side in "Peter", because he doesn't belong in the Blueverse.
This refers to Walter losing his Peter in "Peter".
The mistake and interference happened in "Peter" when The Observer distracted Walternate from finding the cure for Peter and when The Observer saved Walter and Peter from the icy lake.
August is referring to when Walter crossed universes in "Peter" to save the other Peter.
Newton was referring to the events of "Peter" where Walter's Peter died and Walter crossed universes to kidnap another Peter.
Walter was struggling with whether or not to tell Peter about his true origins from "Peter".
Walter opened a door to another universe in "Peter" and the world has been paying ever since.
Walter's Peter died in "Peter".
In "Peter" he literally crossed universes for family.
This is exactly what Peter's biological parents experienced in "Peter".
Peter was right; he was stolen from his real parents in "Peter".
Walter experienced that kind of grief in "Peter".
Walter got angry in "Peter" when Carla Warren spoke that same Oppenheimer line at him.
Later, in the timeline without Peter:
She was right, and those events began in "Peter". This mirrors what Walter said in "Peter".
In the season 4 timeline, Walter tried to save/kidnap Peter just like he did in "Peter", but September never saved them from the icy lake.
Walter made a similar comment in "Peter".
Peter had flashbacks of literally losing Etta in "Transilience Thought Unifier Model-11".
Etta died in "The Bullet that Saved the World".
In "Anomaly XB-6783746" Michael gave Walter back memories of the original timeline and showed him that September's words about "the boy" also referred to Michael. (Image 1)