In our universe, Olivia joins Peter and Walter in search of a strange box that is causing trances and death when opened. Walter attends the reading of Bell's will, and is surprised by what he learns.
Olivia and the Fringe Division team on the other side investigate a series of seemingly random accidents, only to find that they are being orchestrated by a man whose insight into probability allows him to virtually predict the future.
A U.S. Senator is discovered to be a shape-shifter after he is rushed to the hospital, and Olivia worries that her secret will be exposed after Thomas Jerome Newton tries to destroy the evidence.
On the other side a man is rescued from amber quarantine by his twin brother, threatening to unravel the controversial containment protocol with a public outcry. Meanwhile, Olivia submits to Secretary Bishop's experiments attempting to help her cross between worlds.
After a radio transmission erases the memories of more than a dozen people across the East Coast, the Fringe Division finds a bizarre connection to a theory of an ancient race of people on Earth. Meanwhile, Walter grows more vocal in his opposition to Peter's work on the doomsday device.
On the other side, Olivia plots her escape -- while working the case of a serial kidnapper who steals children in order to harvest their youth, and who has a deeply personal connection to Broyles.
Olivia pleads for help in escaping from the other universe and returning home, while on our side her doppelganger puts her final plan in motion after Peter discovers who she really is.
The theft of a heart from a transplant recipient sends the Fringe team in search of a man attempting to rebuild the original donor. Olivia and Peter assess their relationship in light of recent events.
When an Observer brings a dead boy from the past to speak with his father, Walter tries to help the man -- one of his musical idols -- remember the conversation and understand the Observer's ultimate agenda.
When the newly-assembled doomsday machine reacts to Peter's proximity, he and Walter grow more concerned about his relationship to it. But when shape-shifters begin turning up dead, the race is on to crack the other Olivia's database and find the killer.
A scientist is murdered with a powder that disintegrates every bone in his body, leading to Fringe team to a former military officer with a very personal grudge. But when the suspect is injured, the case hinges on one of Walter's former test subjects who may be able to read minds.
Olivia's team in the alternate universe investigates the return of a once extinct beetle, which is being engineered by a scientist using unsuspecting human hosts. Meanwhile, Olivia reconnects with her boyfriend.
While Olivia and Peter attempt to reconcile the emotional chasm between them, the team is called to investigate an apparently haunted apartment building -- leading them to a widow grieving the death of her husband.
In 1985 Walter conducts experiments on a young Olivia Dunham in the hopes of finding a way to cross between worlds and return Peter -- who is convinced that Elizabeth is not his real mother. In the other universe, Walter and his wife grieve the inexplicable loss of their son.
After a pair of gravity-defying thieves are caught at the scene of a crime, the team learns that they are test subjects in an experiment using a super-heavy element. Meanwhile, Walter struggles to move forward without his old partner.
An unexpected visitor inside Olivia's body complicates matters when the team is called to investigate a woman who cannot die, and who is preying upon the suicidal for their soul energy.
In the other universe Olivia is kidnapped by people who accelerate her pregnancy to serve their secret agenda -- unaware that Olivia has a condition that could lead to her death. Secretary Bishop lets Lincoln in on a secret so that he can find her.
With Bell's consciousness inside Olivia and time running out, Walter and Peter attempt to go inside Olivia's mind to find her consciousness and bring her back. Outside, Agent Broyles takes a trip of his own.
After a vortex appears at a remote farm the Fringe team concludes that Walternate has succeeded in activating the device in the other universe. As they struggle to find a way to survive, the other Olivia takes drastic actions.
With Peter unconscious and a freak lightning storm plaguing the Northeast, Olivia teams up with Sam Weiss in the hopes of fixing the machine before it tears the universe apart.
Fifteen years into the future, Peter and Olivia call upon an imprisoned Walter for help in stopping a group of terrorists seeking to accelerate the end of days -- unaware of who is truly responsible.
