After a plane from Hamburg returns with no survivors, FBI agent Olivia Dunham goes after the only person that might shed some light on the incident - a scientist that has been in a mental hospital for the last 17 years.

A woman gives birth to a baby who ages eighty years in just a few minutes. Olivia soon finds connection between the incident and a serial killer she had been chasing.

Following a strange apparent terrorist attack on a bus, Olivia, Peter and Walter find a man who foresees future disasters before they happen.

A strange cylinder which is somehow connected to Walter and a man known as "the Observer" appears from the ground at a construction site.

An elevator crashes, killing a number of people, leading Olivia and the Bishops to a man who appears to unknowingly be able to manipulate electricity.

A number of people in a diner die of radiation exposure and Olivia and the Bishops believe a woman who disappeared two weeks earlier was the source.

An FBI agent experiencing huge pain is brought to a hospital where it is discovered that there is a strange parasite wrapped around his heart.

A young musical genius is kidnapped and Walter recalls a patient at St Clair's who might have been kidnapped by the same people and forced to try and figure out the solution to an equation.

The Fringe division investigate the questionable fall of a Massive Dynamic executive analyst. John helps Olivia crack the case. Peter meets up with an old girlfriend.

After a series of mysterious bank robberies, a suspect is found embedded into a wall, causing Walter to realize that they have not only found a way to literally walk through walls, but are looking for something he's made in the past.

Olivia has been abducted, and Broyles and the Fringe division must investigate her disappearance - while also undergoing an audit by an agent from Olivia's past who bears a major grudge against her.

Two men's brains are liquefied and their only apparent connection is that they both downloaded a huge file before dying.

A man undergoes a painful and ultimately fatal transformation aboard a plane. Meanwhile, Nina Sharp and Broyles reveal more information about John Scott to Olivia.

The team investigates a number of people who died faceless after their orifices are sealed while German agents investigating David Robert Jones' escape question Olivia.

A mysterious child is recovered from a secret chamber that has been sealed for more than a half-century.

A group of animal rights activists are killed in action when trying to flee from a lab animal which doesn't exist in nature.

FBI agent Dunham believes that she is killing random people in her dreams.

Walter examines a number of dead bodies that had the spinal fluid removed. Olivia and Charlie go after the scientist who created the person behind the murders.

The Fringe team investigate a case of spontaneous human combustion. Olivia starts experiencing visions of the alternate Universe. She uses this ability to solve the case.

David Robert Jones comes out of hiding following an attempt on Nina Sharp's life while Walter leaves accompanied by the mysterious Observer.

Olivia is in a car accident, an unusual person is looking for a member of the team, and Walter makes plans for celebrating Peter's birthday.

Olivia and Peter investigate a series of strange disappearances. Olivia hears and sees things from the alternate Universe but fails to realize Charlie isn't really Charlie.

The Fringe division investigates the explosion of a police officer. Olivia starts having visions about her experience on the other side.

Walter analyzes the body of a shapeshifter and then reconnects with an old patient that has the ability to distinguish the people from the other world. Olivia finally recalls her encounter with Belly.

People who once had sleeping problems but were cured by having a biochip inserted in their brain suddenly start to die. Walter suspects the chips were accessed by someone who used mind control.

Strange deaths by a mysterious shadow leads the Fringe Team to the scene where they learn that a Russian astronaut was possessed by an alien and his brother is convincing Dr. Bishop to solve the equation to free him.

The Fringe team investigate a child that is supposed to have mind control abilities. Peter is kidnapped and controlled by him. Walter is extremely worried and afraid he might lose Peter again.

Peter and Olivia look into the latest appearance of an observer, other than September, who kidnaps a young woman. August prevents her from getting on a plane that would crash.

Walter examines some bodies that have been infected with unusual worms. Olivia and Peter run an investigation after speaking with a survivor. Meanwhile Walter demands more independence from Peter.

Olivia and Peter look into some patients of mental institutions that regain their mental state after having gone through a brain intervention performed by a shape-shifter named Newton. This is directly related to Walter and his brain.

A teenager comes back from the dead possessed by a soldier who's gone missing. Walter does an experiment so that the Fringe team can find the soldier's murderer.

When the sheriff of a small town nearby Edina City gives a ride to a boy that is fleeing home, he finds at the police station that the boy has changed to a deformed child. Out of the blue, a group of deformed man slaughter the sheriff and two marshals and bring the boy with them. The Fringe Division is assigned to investigate the case and they head to Edina, here they are received by the local sheriff. They overhear a humming and the sheriff explains that it comes from a military base. Soon Olivia and Peter are attacked by a local in a truck but Peter succeeds to kill him. They bring the corpse to Dr. Bishop that catches a rare butterfly to Astrid that collects the insect. But when he deliver the pot with the butterfly to her, she has turned into a moth. Astrid and Dr. Bishop drive to Edina City and when the moth becomes a butterfly again, Dr. Bishop discovers the secret of Alina City.

After a man spreads a pathogen on the grounds of a company, the Fringe team start an investigation. While stuck inside the quarantine zone, Peter and Olivia try to identify the origin of the pathogen. Outside, Walter tries to find a cure.

A man uses a toxin to target specific groups of people. Walter finds out that the toxin was created by his father in Berlin, long before DNA was even discovered.

An earthquake from the other side is being felt on our Universe as well. Furthermore, the only survivor seems to be from the alternate Universe. Walter attempts to get Olivia to "recover" her ability to make out things from the other side.

Walter flashes back to 1985 while explaining Peter's otherworldly origins to Olivia. Also, Peter's mother is introduced, and details of the neighboring world reaffirms that there is more than one of everything.

The Fringe team investigate a series of mysterious deaths caused by cancer that seems to have been transmitted by touch. Olivia realizes the perpetrator is after the kids who were part of the Coretexiphan trials.

The Fringe team investigate a train full of people who died in a mysterious way. Olivia and the others then meet the astrophysicist who was responsible for the "experiment" and who can reset the time.

The body of a male teenager is found along with shapeshifter marks. Walter examines the embryo of a shapeshifter, that came from the alternate Universe. The team try to anticipate the shapeshifters' next moves.

Walter has just smoked a drug that he calls "Brown Betty" when Olivia arrives at the laboratory to leave Ella with Astrid. The girl asks Walter to tell a story and he creates a detective story where the Fringe Division are the characters.

Peter stumbles across Newton and his people while running away from Walter and the FBI. He teams up with the local sheriff to try and find Newton.

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

Sacrifices will be made and both universes may never be the same again after Walter and Olivia visit the "other side."

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.

Three men abduct and keep a family tied up and gagged in the living room. Two of them dig the basement while the third one watches the family. The men find a box and decide to take a peek inside. The two men and the family immediately die. The Fringe Division investigate the case and Walter performs an autopsy on the bodies. Soon he finds that they die because they have been exposed to an ultrasound signal. Walter receives an envelope from William Bell's lawyer as inheritance and he discloses to Astrid what William left for him. Meanwhile the alternate Olivia and Newton, who had hired the men to find the box and bring to him, plot a scheme to retrieve the box. Then they set in motion a Machiavellian plan.

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.

When the American Senator James Van Horn has a serious car accident, he immediately goes to surgery; however Newton invades the hospital and abducts him shooting the staff. There is a shootout with Broyles and Newton shoots Van Horn in the eye. The Fringe Division finds that the senator is a shapeshifter and they take his body to Walter's laboratory. He finds that it is possible to revive the shapeshifter and recover his data disc. But the alternate Olivia calls Newton and he sends the shapeshifter Ray to the hospital to retrieve the disc. When he returns home, Newton orders him to kill his family. However Ray is connected to his wife and son and refuses to follow the order. What will happen?

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.

The latest broadcast on a numbers station transmits amnesia to its listeners. Walter clashes with Peter over Peter's work on the machine from alternate Earth.

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.

Fringe Division seeks a man who has been harvesting transplanted organs, leaving victims with nearly nullified decay rates. Olivia faces the aftermath of Fauxlivia running her life.

The Observer returns, taking an active role in righting an old wrong (which includes - mundane for him - thwarting a bank robbery).

The Fringe Division visits the Massive Dynamic to see the Doomsday Machine and the presence of Peter assembles the device. He is submitted to tests by Dr. James Falcon against Walter's will. Walter asks Nina to find William Bell's research since he wants to restore his brain to increase his IQ and help Peter. Meanwhile Broyles gives access to the main frame where the alternate Olivia's files are to help the investigation and they suspect there is a mole accessing the information. Soon they find that someone is killing the shapeshifters. Who might be the killer?

When a scientist is killed by a blue dust that grinds his bones up, the Fringe Division investigates the case and suspects a biological attack. The investigation leads them to a former marine that flees but is hit by a car. In coma, Walter recalls his Cortexiphan patient Simon Phillips that is capable to read mind. Olivia, Peter and Walter try to convince Simon, who lives in a deserted area, to help them to locate the terrorists, but he is sensitive to other people since he cannot control his ability. Simon agrees to help and also disclose Peter's real feelings for her.

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.

This follow-up to last season's "Peter" flashback episode revisits a poignant period of time for both the Bishops and Olivia.

The Fringe team attempts to find out what process enables people to become lighter than air and float. Peter and Olivia deal with trust issues in their relationship, while Walter becomes increasingly obsessed with retrieving William Bell's consciousness from beyond the grave.

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.

With the latest estimate that the team has only one day to save Olivia's consciousness, Walter leads a group on an LSD trip into Olivia's mind.

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

Olivia joins forces with the mysterious Sam Weiss in a desperate race against time.

Peter successfully enters the machine with Olivia's help, and is transported to the future days where Walternate's universe is the one which has been destroyed. Walter learns about the origins of the machine and its ultimate purpose and sends back Peter to change his choice but gets lost in time warp.

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.

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 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.

Walter retrieves another video tape from the amber and it leads the team into a pocket universe.

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.

Following the bread crumbs, Olivia collects a truck with cargo from a scrap yard but runs into bounty hunters. Peter, with newly gained powers, has to make a decision.

Walter Bishop opens Pandora's Box. He finds a young Observer once he has hidden in the Pocket Universe.

Young observer remains muted. The team seeks Nina Sharp's help to understand him, but the mission is compromised.

Young Observer's origin is revealed. And his existence and an additional time travel are necessary to change the history of Observers' invasion.

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

As the Observers close in, Walter and September desperately work to complete the plan, but one obstacle after another hinders their progress.

The Fringe team investigate a train full of people who died in a mysterious way. Olivia and the others then meet the astrophysicist who was responsible for the "experiment" and who can reset the time.

Walter flashes back to 1985 while explaining Peter's otherworldly origins to Olivia. Also, Peter's mother is introduced, and details of the neighboring world reaffirms that there is more than one of everything.

As the Observers close in, Walter and September desperately work to complete the plan, but one obstacle after another hinders their progress.

A man uses a toxin to target specific groups of people. Walter finds out that the toxin was created by his father in Berlin, long before DNA was even discovered.

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 and Peter look into some patients of mental institutions that regain their mental state after having gone through a brain intervention performed by a shape-shifter named Newton. This is directly related to Walter and his brain.

Walter Bishop opens Pandora's Box. He finds a young Observer once he has hidden in the Pocket Universe.

When the sheriff of a small town nearby Edina City gives a ride to a boy that is fleeing home, he finds at the police station that the boy has changed to a deformed child. Out of the blue, a group of deformed man slaughter the sheriff and two marshals and bring the boy with them. The Fringe Division is assigned to investigate the case and they head to Edina, here they are received by the local sheriff. They overhear a humming and the sheriff explains that it comes from a military base. Soon Olivia and Peter are attacked by a local in a truck but Peter succeeds to kill him. They bring the corpse to Dr. Bishop that catches a rare butterfly to Astrid that collects the insect. But when he deliver the pot with the butterfly to her, she has turned into a moth. Astrid and Dr. Bishop drive to Edina City and when the moth becomes a butterfly again, Dr. Bishop discovers the secret of Alina City.

A young musical genius is kidnapped and Walter recalls a patient at St Clair's who might have been kidnapped by the same people and forced to try and figure out the solution to an equation.

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.