After stepping into the machine with the help of Olivia, Peter confronts his destiny, and the team finds itself on unfamiliar ground as the true cost of saving the universe becomes clear.

As the disastrous effects of Walternate's plan to destroy our universe start to manifest, Olivia joins forces with the mysterious Sam Weiss in a desperate race against time.

In the Alternate Universe, when Secretary Bishop finally determines a method to refocus the damaging anomalies in his universe onto this one, the beginning of the end for both universes is triggered.

Back "over here", Walter and Peter embark on a desperate mission inside Olivia's subconscious mind, in an attempt to remove William Bell's consciousness from Olivia's body before her soul is lost forever. Meanwhile, Astrid takes care of Broyles when he's accidentally exposed to LSD in Walter's lab.

In the Alternate Universe, Olivia is kidnapped and given a dangerous procedure to accelerate the birth of her child. As the Fringe Division searches for her captors, Lincoln and Charlie learn the truth, that their Olivia and Olivia of the Prime Universe had recently temporarily swapped places, and become distrustful of Walternate's agenda for the future of their world.

When the Fringe team investigates an apparent suicide victim, they uncover a second set of fingerprints leading them to a woman with uncanny characteristics that cannot die. Meanwhile, William Bell, in possession of Olivia's body, promises that no harm will come to her and he will vacate his temporary residence as soon as he finds a suitable host.

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, and Olivia and Peter test the boundaries of their new relationship.

In a flashback to 1985, Elizabeth Bishop, worried that Peter will attempt to kill himself trying to return to what he believes is his home, takes him to Jacksonville where Walter is conducting Cortexiphan trials on children. Walter discovers that young Olivia has been able to cross to the "other side", and sees it as a way to return Peter to the Alternate Universe where he belongs.

A series of Fringe events leads the team to the home of a woman grieving over the death of her husband, while Peter and Olivia make an effort to repair the emotional rift between them.

As the story shifts back to life in the Alternate Universe, the absence of Colonel Broyles shifts the dynamic of the Fringe Team as they investigate a bioterrorist armed with an insect that has a taste for human flesh. Meanwhile, alternate Olivia is reunited with Frank, and Walternate remains determined to save his world but discovers there are certain lines he will not cross.

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.

When the Fringe Team visits Massive Dynamic's assembly of 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.

An Observer unexpectedly makes contact with the Fringe Team to help rectify a mistake. Meanwhile, Walter befriends Roscoe Joyce, keyboardist for Walter's favorite 1970s band, "Violet Sedan Chair." Walter will learn the real reason the band broke up, and realize that the lives of these two men, who have never met before, overlap in strange and unexpected ways.

When the Fringe Team investigates a case where a victim had his heart extracted, it's just the beginning of a series of mysterious and reanimating events tied together. Meanwhile, Peter comes to terms with the reality of his relationship with Olivia.

Peter grapples with the aftermath of recent events as Olivia desperately searches for an ally. Meanwhile, the Fringe Team discovers a critical device that communicates between universes.

When a serial kidnapper "over there" strikes again, the emotional and familiar case hits home for Colonel Broyles, sending a determined Olivia to uncover additional details about the abductions. In the meantime, Olivia fights on and reunites with Henry to enlist his services on an intense and covert mission to return home.

Back "over here," Fringe Division investigates a bizarre phenomenon when 15 people up and down the Eastern Seaboard, all suffer retrograde amnesia from listening to their shortwave radios on the same frequency. Much to Walter's dismay, Peter presses on with piecing together the mass destruction device. Just as alternate Olivia and Peter's chemistry deepens, the anticipation of Olivia's return escalates.

In the Alternate Universe, Fringe Division investigates a shocking breach of security when a twin frees his brother from a quarantined Amber area. As the team sets out to crack this sophisticated case, Walternate begins experimenting with Olivia's ability to cross between universes. Meanwhile, visions of Peter continue to haunt Olivia about returning to the "other side".

Back "over here", Newton is concerned about the consequences of a distressing development involving a high-ranking official, and 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.

In the Alternate Universe, timing is everything as a series of anomalous events lead 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.

The story alternates to the Fringe Team "over here" where they investigate a mystifying case that people are discovered in a trance-like state that ultimately leads to their death. The unlikely suspect: a mysterious box. As Peter and Walter collaborate to study its properties, alternate Olivia turns up the heat on an unsuspecting Peter. Meanwhile, key information about the future is revealed when Walter and Nina meet at Massive Dynamic for the emotional reading of the last will and testament of William Bell.

After the extraordinary turn of events that shockingly left an imprisoned Olivia stranded in the Alternate Universe, she fights to find a way home. Meanwhile, Peter and Walter try to move on with their lives unknowingly alongside the Olivia from the other side.