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.

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.

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 Peter disappears, Walter seeks solace in marijuana. In that state of mind, he and Astrid babysit Olivia's niece Ella, and Walter tells her a fairy tale of a gentle inventor, a stolen glass heart, a detective looking for true love, and a mysterious group of bald men known as the Watchers.

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.

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 experiments over there more with Olivia as she reenters the tank. Meanwhile, visions of Peter continue to haunt Olivia about returning to the “other side”.

Back “over here,” the 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.

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

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.

As the story shifts back to life “over there,” the absence of Colonel Broyles changes 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 her beau, and Walternate remains determined to save his world but discovers there are certain lines he will not cross.

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.

The intensity of life “over there” accelerates as Olivia finds herself in mortal danger.

In an effort to remove Bell's consciousness from Olivia's mind, Peter, Bell, and Walter enter Olivia's subconscious and attempt to find her personality, which has hidden itself somewhere safe.

The beginning of the end is triggered when Walternate finds a way to wreak havoc “over here."

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.

In an effort to find a way home, Peter journeys to the Other Side to confront Walternate. However, his venture proves more dangerous than expected. Meanwhile, Olivia receives a dire premonition from an Observer.