Olivia returns home via a car accident that leaves her with no memory of her experience, and the team tracks an assassin who can shape-shift in order to get close to his target.
A series of disappearances in a small town leads to an underground creature and a man who may be protecting it. Agent Dunham begins to experience heightened senses as a result of her ordeal.
After a Philadelphia police officer is converted into a human bomb, the team must identify the trigger man and other walking weapons. Meanwhile, Olivia grows frustrated with her unorthodox rehab.
A group of shape-shifters steal cryogenically frozen heads, and Walter turns to a very special former patient to try and identify them. A flood of memories brings Olivia a dire warning from William Bell.
When patients in a sleep study at a Seattle clinic experience frightening visions that cause them to attack their co-workers, Dunham and the team suspect that someone is hacking into their brain implants to try and perfect mind control.
The team chases two men who appear to have kidnapped the son of a Massive Dynamics employee, and who can control people with a thought. But it soon becomes clear that all it not as it seems.
A second Observer surfaces, breaking their pattern of non-involvement by abducting a young woman in broad daylight -- for reasons that surprise even his own people.
When a group of Chinese refugees are killed by enormous parasites inside them, the trail leads to a scientist who is smuggling the creatures into the country to harvest something from them.
Unauthorized brain surgery on a mental patient renders him completely cured, and sends the Fringe team on a hunt for an enemy who is attempting to reconstruct a long-buried secret about the other world.
A teenage girl's miraculous resurrection has consequences when she begins spouting secret codes and speaking Russian, inexplicably connecting her to a missing Navy officer.
A runaway boy inadvertently causes the deaths of three state troopers, bringing the team to a remote town where the military once conducted experiments -- and where the locals are trying to keep a very personal secret.
The death of a man inside an office building triggers a quarantine by the C.D.C., locking Peter and Olivia inside amidst the outbreak of an intelligent virus that is millions of years old.
The targeted murder of a Jewish family points to the research of Walter's own father, who developed a programmable pathogen while working as a spy in the Nazi regime.
A seismic disaster in Manhattan convinces Walter that the two universes are colliding, bringing the team to an abandoned daycare in Jacksonville -- where Olivia must relive the experiments to which she was subjected as a child, in the hopes of preventing another disaster.
Walter tells the story of his 1985 experiments in which he attempted to save his son's life by copying the advancements of the other universe -- where the lives of both Peters were on the line.
A woman's sudden death of what appears to be aggressive and contagious cancer leads the team to a man who is targeting former Cortexiphan test subjects. Meanwhile, Olivia struggles over whether to keep Walter's secret.
The team chases a brilliant scientist who has devised a way to travel in time -- but at the cost of draining the life of every living thing in his path. Walter makes a decision about Peter.
A team of shape-shifters arrive to help Thomas Jerome Newton enact a new plan to bring something -- or someone -- over from the other universe, while Walter examines an undeveloped shape-shifter embryo.
Asked to help babysit Olivia's niece, Walter tells her a musical noir tale about a private detective hired to investigate the theft of a powerful glass heart ... by one Peter Bishop.
Peter finds himself in the middle of a murder investigation after a small-town waitress turns up dead, evidently at the hands of Thomas Jerome Newton. In Boston, Walter struggles to maintain his independence without his son.
After Peter's whereabouts are discovered Walter and Olivia ask a group of Cortexiphan subjects to help them cross over to the other universe, where an alternate Fringe Division is working for the Secretary. Meanwhile, Peter meets his real mother.
Olivia approaches her other self for help in rescuing Peter, who has been asked by his father to aide in a project with far-reaching consequences. Walter and Bell reunite to find a way to open the door back home.
