Olivia, captured on "the other side", attempts to escape and return to her home world. Meanwhile, Walter attempts to reconcile with Peter, but they are both unaware that the Olivia with them is not the real one.
The other Earth's Fringe team investigates people who have been put into an unbreakable trance by a mysterious box. Meanwhile, Walter and Nina attend the reading of the last will and testament of William Bell.
In this case, timing is everything as a series of anomalous events lead the Fringe Division to investigate deadly incidents with unimaginable coincidences. As the freaky crimes continue to occur, the team is in a race against time to protect potential victims. Meanwhile, Secretary Bishop shares his strategic plan with Colonel Broyles as Olivia grapples with a heart-stopping vision.
Newton, concerned about the consequences of a distressing development involving a high-ranking official, is forced to call to action a sleeping shapeshifter. As Walter and the rest of the team gather evidence, they move the investigation to Massive Dynamic, where Olivia goes on high alert and Walter finds himself in a perilous situation.
On the Other Side, a man sets out to free his twin brother from an amber quarantine zone. Meanwhile, Walternate begins the experiments to duplicate Olivia's ability to transition safely between worlds.
On This Side, 15 people across the Eastern Seaboard suffer amnesia after hearing the same radio frequency, and the Fringe team is called in. Meanwhile, Peter continues to work on the Other Side's mass-destruction weapon and pursue his relationship with "Olivia," unaware of who she truly is.
On the Other Side, Olivia tries to handle a series of kidnappings with connections to Colonel Broyles, while turning to a hidden ally to find a way back home.
Peter struggles to cope with recent events; Olivia searches for an ally; a device that communicates between universes is discovered.
While the Fringe team investigate the case of a man who had his heart extracted, Peter deals with the revelation of who he's been dating for the last few months.
A mistake is made which prompts one of the Observers to reach out to the Fringe Team to help make things right. Also, Walter meets one of his musical heroes.
When the Fringe Team visits Massive Dynamic's assembly of the doomsday device, Walter becomes greatly concerned for Peter's well-being, so he turns to Nina for added brain power to understand the relationship between Peter and the superweapon. Meanwhile, the discovery of a dead body triggers an intense investigation.
When a scientist falls dead after ingesting a lethal cloud of blue powder and his bones disintegrate in his body, the CDC suspects a biological attack. As the Fringe Team investigates further, a subject from Walter's past with ties to Olivia reluctantly agrees to assist with the intense case.
A terrorist Over There arms himself with an insect that enjoys human flesh; alternate Olivia is reunited with her boyfriend; despite remaining determined to save his world, Walternate discovers there are lines he won't cross.
The team is lead to the home of a grieving widow; Peter and Olivia try to fix their relationship.
In a follow up to last season's "Peter" flashback episode, the all-new "Six Months Later" episode revisits a poignant period of time for both the Bishops and Olivia.
The Fringe Team investigates a group of thieves who can break the laws of gravity. Meanwhile, Walter attempts to delay the damage he's caused to the fabric of the universe.
Fingerprints at the scene of an apparent suicide lead the Fringe team to a seemingly immortal woman.
On the Other Side, Bolivia discovers that there are men that will stop at nothing to obtain her unborn baby, which may hold the secret to safe travel to This Side.
The presence of William Bell's consciousness in Olivia's body threatens to displace her mind permanently, unless Walter and Peter take extreme measures.
The beginning of the end is triggered when Walternate finds a way to wreak havoc "over here".
Olivia joins forces with the mysterious Sam Weiss in a desperate race against time.
Peter is thrown forward into the year 2026, where he is forced to face the consequences of his decision to activate the Machine and destroy the Other Side. Meanwhile the Observers gather at Liberty Island to witness the outcome as worlds collide.