In an effort to find a way home, Peter journeys to the Other Side to confront Walternate. His venture proves more dangerous than expected. Meanwhile, Olivia receives a dire premonition from an Observer.
Our Astrid gets an unexpected visit from her Alternate, while Peter and Olivia track a killer using a toxin that has yet to be invented. Both universes collide in a case that pushes the boundaries of what is possible.
As the Fringe team investigates a mentally ill patient who appears to be connected to a series of murders, frightening clues and connections are uncovered that lead Olivia to some shocking revelations.
The team hunts a killer who seems to be targeting couples - dehydrating the husbands and smothering the wives - with a very unusual motive. Peter and Olivia try to come to terms with her surfacing memories and emotions from the original time-line and the fact that she's beginning to forget parts of her life from the new time-line. Meanwhile, Walter and Peter try to figure out September's disappearance from the lab the week before.
An incident occurs that bears remarkable similarities to an early case in the original time-line, though there are some notable differences. Olivia's memories of the new time-line continue to fade, and Broyles places her on temporary leave, but her memories of original case assist Peter and Lincoln. Lincoln gets infected with a virus and Walter must figure out a cure.
Lincoln, feeling out of place on the team now that Peter is back and Olivia is beginning to forget him, volunteers to take liaison duty for the day, and joins the alternate Fringe team in the search for a vigilante killer with some unusual abilities. He bonds with Fauxlivia and Alt-Astrid, and he and Alt-Lincoln compare life stories to figure out the way in which their two lives diverged, only to find that they're not so different as they initially seem to be. Alt-Lincoln still suspects that there may be a mole in his team working for David Robert Jones.
When both of the two identities of people from the separate worlds experience the same catastrophes from incidents in just one world, the two Fringe teams collaborate. Walter crosses over to find that Jones is behind the phenomena, but to what ends? Also, the investigation into the Fringe-mole continues.
In the future, the Observers rule and humans that survived the purge serve them. There are still a small number of people fighting for the resistance, and one of them has discovered one of the original Fringe team in the form of Walter protected in amber. As the resistance attempts to get rid of the Observers, we get a glimpse into a rather dark dystopian future.
Multiple earthquakes are happening simultaneously around both our world and the alt-world. Walter believes he knows what the end game of David Robert Jones is. And, even though Olivia has a connection to one of the players Jones is using that she thinks she can utilize, in the end it leads to some hard decisions for the Fringe team.
In 2036 the Observers have begun terraforming the earth. Walter has a plan to defeat them but his capture by the Observers may be problematic. Peter searches for Olivia who is encased in amber to reunite his family
Harvard is now a secret observer installation. But Walter remembers a secret entrance that will give him access to his lab and perhaps a way to recover his plan to rid the world of the Observers. Olivia gets a distasteful look at Observer interrogation methods as Etta questions a loyalist security guard in hopes of finding a way to restore power to the lab. As Olivia and Etta disagree on how to dispose of their captive they gain a greater appreciation of each others character.
After piecing together the videos of Walter's plan Olivia, Peter, Walter and Etta head to rural Pennsylvania to retrieve essential materials for the plan. There they encounter a group of archivists who may be able to help with some local obstacles.
An act of kindness by Peter draws attention to Etta. The Observers deal with an infiltrator and we learn of a resistance leader known as the Dove and coincidentally that that Phillip Broyles is working with the Observers.
Peter in the grip of mourning Henrietta's death, resorts to extreme measures in order to gain powers that will allow him to surpass those that the Observers possess - and hopefully and eventually find a means to defeat them.
The team retrieves another cassette tape from the amber. With Nina Sharp's help, the team recovers 2 cylinders. While Peter works independently to score a minor victory against the Observers, Olivia ascertains Peter's secret.