Walter and Olivia travel to the parallel universe, and the anticipated face-off between Walter Bishop and William Bell occurs.

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 the alternate universe, Olivia and the Fringe Division investigate weird accidents with casualties apparently with no connection except ballpoint pens found on the spots. But when Olivia sees the freak Milo, she suspects that he might be the responsible. Olivia and Charlie investigates and soon they follow a lead to a hospital where they learn what happened to Milo.

In the parallel universe, a man releases his twin brother from an area in quarantine, removing him from the amber. The Fringe Division investigates the case that is classified since The Secretary does not want that the population learns that the family and friends are alive enclosed in amber. Olivia interviews Matthew, who is the twin brother of Joshua, and suspects that he is lying. Meanwhile The Secretary lures Olivia and convinces her to be submitted to n experiment while she has daydreams with Peter.

In the parallel universe, a kidnapper called The Candy Man abducts the boy Max. The Fringe Division is assigned to investigate the case and soon Olivia learns that Broyles' son Christopher was also abducted two years ago and returned blind and with physical deficiencies to his family. She asks Broyles to interview Christopher to see any missing information but Broyles does not accept; however his wife agrees that Olivia meets Christopher. Olivia recalls an old case and guess whether The Candy Man would be draining the pituitary gland from the children to use the hormones to stay young. Based on the received information, Olivia and Broyles meet Reverend Marcus, who was a former physician and now is responsible for a church where they believe the abductor is a member. Meanwhile Olivia meets Henry and asks him to help her to go to the Liberty Island to use the deprivation tank to return to her universe.

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.

In the parallel universe, the Fringe Division investigates a case where a man was eaten from inside out by an extinct beetle. Meanwhile Broyles is considered missing and Lincoln is the head of the division. Olivia welcomes her boyfriend Frank that has just returned from Texas and wants to propose her. When a second man is also eaten by the insects, the Fringe Division identifies the scientist Dr. Armand Silva that was researching the cure for avian flu using this insect. Lincoln and Olivia goes to his address and split. Lincoln is locked in a refrigerating chamber while Olivia is brought to his laboratory, where she drinks water. Dr. Silva explains that only human beings are capable to host the beetle and a new cycle has just began. Olivia concludes that she has been infected by the insect and despairs.

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 Fringe team hopes to alleviate Olivia's possession quickly, Agent Lincoln Lee of the F.B.I. meets them for the first time over the curious case of what he calls "the compassionate soul vampire."

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

The beginning of the end is triggered when Walternate finds a way to wreak havoc "over here" - putting both universes in jeopardy.

Just one week after Peter Bishop saved the day and was then wiped from existence, the truce between the two worlds remains intact and the two sides are forced to work together. Lincoln Lee joins the Fringe team as they investigate a shapeshifter case that is very personal to him.

The "Over Here" team helps the "Over There" team look into a string of murders in which the victims are frozen from the inside out.

Two bullying boys die and exhibit an alarming state of rapid decay. Their would-be victim, an extremely lonely boy, has a remarkably unusual connection to the killer. Meanwhile, Walter fears for his sanity.

Olivia is haunted by an energy force that attracts metallic objects and Walter recalls his subject 9 that has the ability for astral projection. They chase the patient Cameron Jones but they conclude he is not the responsible for the energy force. However Cameron faces the force, something happens.

Shapeshifter Nadine hunts down a former Massive Dynamics scientist to help rectify her cellular translucence. Meanwhile, Peter finds conversing with Walter difficult.

The Fringe Division begins to wonder if Peter or his presence is behind a series of time displacements between 2007 and 2011. Walter imposes a self-exile to his bedroom.

A manhunt begins for an invisible killer who steals human pigmentation.

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.

Peter's visit to the other side intensifies as the alternate Fringe division faces off against a formidable new foe in a harrowing race against time.

Olivia continues to cope with the ominous warning from the Observers, while Peter and the team track a girl with the mysterious ability to predict death.

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.

Peter, Olivia and Walter come face to face with a mysterious and terrifying Fringe event as they get trapped in a town that there's no escaping.

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.

Peter and the Fringe team take extraordinary measures in an attempt to stop life-threatening events.

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 travels to the other side to join forces with Faulivia and his own alternate, as they search for a vigilante with a surprising connection to the shapeshifters.

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.

A mysterious Fringe event, causing people to spontaneously combust, forces Walter to revisit his painful past while the Fringe team faces off against David Robert Jones to save the world - or worlds.

In the Season Four finale, the Fringe team is pushed to their breaking point as they desperately attempt to prevent a catastrophic event threatening the lives of everyone.

As the Observers evaluate Michael and begin to learn of his abilities and potential, Olivia embarks on a dangerous quest to rescue him.

Lincoln travels to the other side to join forces with Faulivia and his own alternate, as they search for a vigilante with a surprising connection to the shapeshifters.

Just one week after Peter Bishop saved the day and was then wiped from existence, the truce between the two worlds remains intact and the two sides are forced to work together. Lincoln Lee joins the Fringe team as they investigate a shapeshifter case that is very personal to him.

As the Fringe team hopes to alleviate Olivia's possession quickly, Agent Lincoln Lee of the F.B.I. meets them for the first time over the curious case of what he calls "the compassionate soul vampire."

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.

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

In the parallel universe, the Fringe Division investigates a case where a man was eaten from inside out by an extinct beetle. Meanwhile Broyles is considered missing and Lincoln is the head of the division. Olivia welcomes her boyfriend Frank that has just returned from Texas and wants to propose her. When a second man is also eaten by the insects, the Fringe Division identifies the scientist Dr. Armand Silva that was researching the cure for avian flu using this insect. Lincoln and Olivia goes to his address and split. Lincoln is locked in a refrigerating chamber while Olivia is brought to his laboratory, where she drinks water. Dr. Silva explains that only human beings are capable to host the beetle and a new cycle has just began. Olivia concludes that she has been infected by the insect and despairs.

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.

A manhunt begins for an invisible killer who steals human pigmentation.

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.

Two bullying boys die and exhibit an alarming state of rapid decay. Their would-be victim, an extremely lonely boy, has a remarkably unusual connection to the killer. Meanwhile, Walter fears for his sanity.