Walter explains to Olivia how Peter came from the alternate universe, and how he was willing to risk everything to save his son.

When the passengers on a commuter train all die instantly of massive power loss, the team realizes that a time traveler is responsible... and that it may be impossible to apprehend him. Meanwhile, Walter writes a letter to Peter explaining his origins.

The Fringe team engages a desperate plan as Olivia embarks on a dangerous and otherworldly journey into the unknown. Then, Peter, Olivia, Walter, Astrid and Broyles face off against the Observers in one final and extraordinary battle for the fate of mankind.

Walternate asks Peter to help complete the device, while the fugitives Walter and Olivia are reunited with William Bell. Meanwhile, the alternate world's Fringe Division closes in on the invaders, and Peter must make a life-shattering decision.

The teams on both sides fight on for a common cause, and there are shocking developments tied to the Cortexiphan children.

Setting the stage for the dramatic and revealing first season finale are a sudden and unexpected attack on someone with close ties to Fringe Division, the return of bioterrorist David Robert Jones and the inexplicable disappearance of Walter. Find out more about the mysterious events surrounding our trio when questions are answered, observations made, loyalties are tested and the elusive William Bell is finally introduced.

As the thrilling third season propels to a climactic close, questions will be answered as Peter confronts his destiny and a beloved team member meets an untimely demise.

German agents question Olivia about her "relationship" with David Robert Jones, an escaped prisoner questioned months earlier now living in the U.S. The Fringe Division investigates a threatening case where victims die faceless from their orifices suddenly sealing.

Olivia begins to regain her memories of her encounter with William Bell. Meanwhile, the "First Wave" tries to collect the head of their leader, and the traitor makes his move.

The Observer chooses to do something other than observe, abducting a college student. However, when the Fringe team investigates, they discover that there is more than one Observer.. in both time and space.

Two teenagers turn up dead and evidence indicates that shapeshifters have borrowed their forms for some special mission. While the Fringe team tries to track them down, Peter grows increasingly suspicious of Walter's secretive behavior.

When Walter discovers that Peter has traveled to the alternate universe, he and Olivia gather three cortexiphan children with the ability to transition safely between the two Earths.

Peter struggles to cope with recent events; Olivia searches for an ally; a device that communicates between universes is discovered...

As devastation spreads on This Side, Walter, Olivia, and Sam Weiss struggle to find a way to stop the destruction and discover that one individual has the ability to allow Peter access to the Machine.

Peter, Olivia, Walter, Astrid and Broyles face off against the Observers in one final and extraordinary battle for the fate of mankind.

The arrival of a building from the alternate Earth causes massive death and destruction to those occupying the site at its arrival. Walter soon realizes that to equalize the balance between universes, another building will have to disappear... along with everyone inside it. The team's only chance is for Walter to activate Olivia's long-buried ability to distinguish objects from the other universe.

The Fringe team must find the pattern between tragic time loop anomalies. As the investigation unfolds, Raymond, an electrical engineer, and his wife Kate, a professor of theoretical physics, enter the picture.

The Fringe team is running out of time to save the world. As Peter, Olivia, Astrid and Walter scramble to tap into a key piece of the puzzle to defeat the Observers, they enlist the aid of Nina Sharp who conjures up a plan.

The team is called in on a bank robbery similar to other robberies in the area but this particular bank has the thief embedded inside the steel wall of the bank. Somehow the thieves have found a way to defy the law of physics. Walter thinks they're after something of his. As the investigation deepens it all comes to a head when the thieves ambush one of the team.

Olivia's sister, Rachel, visits; Walter, Peter and Olivia try to solve the murder of an esteemed epidemiologist, which may be linked to Olivia's abduction.

Olivia experiences "awake dreams," seeing elaborate visions of things not really there. While investigating a disturbing case of a woman who apparently spontaneously combusted, Olivia throws caution to the wind and explores her unexplained visions further. Meanwhile, Walter discusses key information about the Z.F.T. manifesto, and Peter reveals a secret that yields unexpected results in the case.

An amnesiac Olivia makes her return from the parallel universe, while Walter, Pete, and Junior Agent Amy Jessup investigate the mysterious death of a man found with puncture holes in his palate. Meanwhile, Broyles defends the Fringe Division from a Senate subcommittee threatening to shut the organization down.

After the extraordinary turn of events that shockingly left an imprisoned Olivia "over there," she fights to find her way home. Meanwhile, Peter and Walter try to move on with their lives unknowingly alongside alternate Olivia.

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.

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.