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

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

Peter teams up with a local law enforcement official, Sheriff Mathis, on a serial murder investigation with ties to Newton. Meanwhile, Walter copes with the possibility of being sent back to St. Claire's, and someone from the "other side" pays a visit.

While Walter deals with some very upsetting news, he tells Olivia's niece, Ella, a fairy tale that includes musical performances by Olivia and Agent Broyles.

When two teenagers on a date turn up "dead" at an abandoned warehouse with the three puncture wounds to the soft palate - a trademark of the shape-shifters - the Fringe team investigates the evidence as well as the motives of mastermind Newton. Upon discovering a shape-shifting embryo, Walter returns to the lab to conduct further analysis, and Olivia and Peter head to Massive Dynamic for answers. Meanwhile, Peter reveals a family secret to Olivia as Walter struggles to recall what Newton knows about "building a door".

When passengers aboard a commuter train appear to have died a still death, it seems that a switch was flipped because all cell phones, mp3 players, laptops, batteries and bodies have been drained of power. As the Fringe team assembles at the bizarre crime scene, Peter remains suspicious that something is amiss with Walter, who is struggling to keep the unimaginable a secret. When the investigation leads them to Alistair Peck, a very powerful man who has tremendous energy with severe consequences, an ironic set of circumstances surface.

When a perfectly healthy woman is found dead from a disease she never had, the Fringe team investigates the origin of this inexplicably fatal condition before it claims more innocent victims. Meanwhile, Olivia struggles to keep Walter's secret from Peter, and her loyalty to Peter is tested as time goes on.

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.

A Manhattan office building is rattled to its core, leaving only one survivor, and the Fringe Team examines his otherworldly origins. Once Walter surmises that what shook the building was something he discovered many years ago, the team races to Jacksonville to help Olivia reconnect with her past in order to avert what they fear will happen next.

Investigating a lethal toxin that was unleashed on a wedding ceremony as a weaponized experiment, Walter uncovers an alarming formula that reveals a link to the Bishop family tree.

Investigating a lethal virus, Peter & Olivia become isolated inside a quarantined building and face life-threatening circumstances - while the rest of the team tries to save them.

Following an unexplained attack involving disfigured humans, the Fringe team visits Edina City, a small town in upstate New York to uncover leads surrounding the bizarre case. When it's determined that these disfigured people have managed to hide themselves for a while and they'll do just about anything to keep it that way, the investigation takes an unexpected turn.

After a teenage girl is pronounced brain dead, her mother takes her off life support, but when doctors operate quickly to remove her organs, the deceased girl shockingly awakens screaming an alphanumeric code. Equally as perplexing as her resurrection is that she is now somehow able to speak Russian and possesses classified information only a high-ranking soldier would know. While the girl's mind-bending condition intensifies, Walter dusts off some old lab videos and hypothesizes the unthinkable, sending Olivia and Peter to investigate the bewildering case.

A Fringe investigation is triggered when a patient at a mental institution undergoes an impromptu brain surgery that unexpectedly leaves his brain exposed. Despite the unthinkable circumstances, the patient's condition and sanity miraculously improves before the Fringe team arrives. While Agent Dunham and Peter scan surveillance tapes, Olivia recognizes a familiar face as Thomas Jerome Newton, the leader of the "shape-shifters." As similar cases stack up, the Fringe team heads back to the lab to determine exactly how patients are being cured.

After a cargo ship runs aground, the shoreline becomes littered with bodies hosting a giant squid-like creature. The Fringe team descends upon the scene to examine the bodies and discovers that the mysterious organisms are actually giant parasites. As the investigation unfolds, Walter heads back to the lab while Olivia, Peter, and Broyles race against time when they realize the case has ties to a threatening organization.

When a peculiar abduction occurs in Boston, the Fringe team uncovers details of the mysterious man known as The Observer. While the bizarre case takes center stage, Walter, Peter, and Astrid hit the lab to analyze unusual evidence and deal with Walter's insatiable appetite for a milkshake.

When a kidnapping rapidly escalates into a hostage situation in New York, local authorities close in on the suspects only to discover a mysterious force with mind-blowing consequences. As the investigation intensifies, the mystery and threat deepen to unimaginable proportions when the Fringe Division connects a link between the kidnapping and Massive Dynamic.

The Fringe Division is flummoxed by a case where victims are inexplicably disintegrated into ash. The shadowy case casts light on Special Agent Phillip Broyles' past and leads the team to the possibility of foreign Fringe Science. Emotions run high as the alarming events stack up and revealing personal details emerge about the leader of the Fringe Division.

The Fringe team travels cross-country to Seattle after learning of a mysterious incident involving a man who attacked his boss because he believed he was an evil ram-horned creature. As these puzzling occurrences continue, the team tirelessly explores strange and creepy links to dreams. In pursuit of additional information, Agent Broyles has a disconcerting meeting with enigmatic Massive Dynamic executive Nina Sharp that leads the investigation in an unthinkable direction.

Recuperating from the traumatic and alarming meeting with Massive Dynamic founder William Bell, Agent Dunham consumes a powerful fringe concoction that Dr. Bishop prescribes to stimulate her memory. Meanwhile, the Fringe Division investigates a series of robbery cases that are tied to shape-shifting. As clues are tracked and memories are jogged, another woman experimented on by Dr. Bishop is introduced and a flashback reveals more about Olivia's visit to the Alternate Universe.

Peter, Walter, Olivia, and Broyles pursue a strange and deadly occurrence in Philadelphia where a bomb blew up inside a train station but left no trace of any explosive device. The perplexing and unexplained set of circumstances returns Walter to the lab to closely examine the human remains, where he uncovers an unlikely energy source that triggered the explosion. With the explosive threat of more bombs and links to a classified military project, the intense investigation leads Olivia and Peter to Iraq.

A highway construction worker in rural Pennsylvania goes missing when he is mysteriously drawn into an underground tunnel filled with human remains. Meanwhile, back in the lab, Walter is attempting to simulate travel between realities on frogs. When it's discovered that the disappearance in Pennsylvania is not an isolated incident, the Fringe team travels to the crime scene to unearth evidence.

After her trip to an intriguing Alternate Universe to meet Massive Dynamic founder, William Bell, Olivia's shocking return to this reality pits the team unknowingly in a race against time with an ominous adversary. Pursing information about Olivia's time in the Alternate Universe, this new adversary will stop at nothing to discover what Olivia has learned, threatening the lives of anyone who gets in their way.