Masterminded by J.J. Abrams and producing partners Alex Kurtzman and Roberto Orci, Fringe revolves around the FBI's "Fringe" division. Using "fringe sciences," the division investigates cases and incidents that can't be explained by normal scientific reasoning, from psychic powers to parallel universes. Headed up by Director Phillip Broyles, the team consists of the steely Agent Olivia Dunham, who has latent paranormal abilities; the eccentric and open-minded Dr. Walter Bishop; Walter's rebellious son, Peter Bishop, with whom he has a rocky relationship; and the sweet-natured Agent Astrid Farnsworth, who acts as Walter's lab assistant and handler.
The team are unofficially watched over by the enigmatic Nina Sharp, the head of an advanced engineering company called Massive Dynamic, which was founded by Walter's late partner, Dr. William Bell (played by Leonard Nimoy) and also by a group of black-suited bald men known only as "Observers" (akin to Marvel Comics' cosmic Watchers).
Fringe ran for five seasons from 2008 to 2013 and grew a small but dedicated fanbase. While the series got off to a slow start with the monster-of-the-week format of its first season, later seasons built a compelling overarching storyline around the existence of the Other Side - an alternate universe almost identical to our own, but with some key differences.
The first season finale of Fringe contains the biggest revelation of the whole show: the existence of a parallel universe. The monumental discovery happens when Olivia is somehow transported to an alternate version of New York after an accident. Newspapers describe President Obama moving into a newly built White House and photographs reveal the still-standing Twin Towers.
Walter tells Peter that he once "lost something very dear to him" and replaced it with a version taken from another reality. We later see Walter standing in front of a gravestone that reads, "Peter Bishop, 1978-1985," implying that the "something" was his own son.
The guest appearance of Leonard Nimoy as William Bell in the episode was something co-creator J.J Abrams lobbied for after working with the sci-fi icon on his Star Trek reboot.
After the idea was first teased in the Season 1 finale, the second episode of Season 2 confirms that Peter is, in fact, the other universe's Peter. He was switched by Walter after the loss of his universe's Peter in 1985. Walter explains to Olivia that the discovery of the parallel world gave him hope that he could heal his son's life-threatening genetic disorder.
The prime universe's Walter intended to bring alternate Peter to his world in order to treat him after his own Peter perished. However, after seeing his wife's delight at getting her son back, he couldn't bring himself to go through with his plan. Rather than allowing her to grieve, he kept Peter in this universe. Walter also tells Olivia that taking Peter from his original universe created a fracture between worlds that will lead to a catastrophic event known as the "storm."
Earthquakes happening across Earth in the prime universe and the Other Side spur both Fringe teams to work together. Villain Robert David Jones is to blame. Using what they learned about Westfield's "eye of the storm" safe zone in a previous episode, the teams determine that Jones has found a similar hiding place to survive. With the help of Agent Lincoln Lee, Olivia identifies the Cortexiphan subjects are at the center of each earthquake.
The teams are left with no choice but to wipe out the bridge between their worlds, leading to the groups parting ways. Lee decides to stay on the Other Side because he feels like he belongs there, while Walter and Walternate have a frank and touching heart-to-heart about the bad blood between them and their shared love for their son(s). Walter is concerned that the loss of the bridge will take Peter with it and is relieved when this turns out not to be the case.
In the two-part Season 2 finale, the veil between the prime universe and the Other Side is growing thinner. One of the Observers informs Olivia that Peter is destined to bring about the apocalypse. This prophecy seems to be on point, as Olivia and Walter find out that Peter has decided to go back to his original universe with his alternate father, nicknamed "Walternate."
After crossing over with two of her fellow Cortexiphan testers, Olivia realizes their mission will be harder than expected, as Walternate just so happens to be the Secretary of State. In the second part, Olivia's run-in with her own alternate, "Fauxlivia," leads to her doppelganger correctly observing that she has romantic feelings for Peter. When she finally admits this to Peter, he regains a sense of attachment to the universe he grew up in and agrees to go back home.
Unfortunately, the Olivia that goes back with him is actually Fauxlivia, while the prime Olivia is left imprisoned by Walternate on the Other Side.