While the two universes initiate an uneasy alliance, Olivia partners with Lincoln Lee to search for a serial killer with translucent skin. Peter's absence from the new timeline has repercussions for those who depended on him.
After a serial killer strikes in the parallel universe, the alternate team asks for Olivia's help recruiting the suspect's double from our world -- where he is a forensic psychologist specializing in serial killers.
The death and rapid decomposition of two school bullies leads the team to a dangerous entity and the 12-year-old boy who seems to have a connection with it. Meanwhile, Walter is increasingly troubled by his hallucinations.
Walter tries to help Olivia identify an energy apparition that is targeting her, leading them to a man who was once a part of the Cortexiphan experiments. Meanwhile, Walter learns that Olivia is about to decide whether or not to have him recommitted to St. Clare's.
When an advanced shape-shifter goes after a scientist it believes can stabilize its cellular structure, Peter takes advantage of the situation to prove his usefulness -- and to ask Walter for help understanding why he is there.
The team suspects that Peter's presence may be to blame for a sequence of spherical time distortions, but the investigation leads to a brilliant theoretical physicist and her desperate husband.
After an attack by an invisible assailant leaves the victim a ghostly white, the Fringe team discovers a man with a unique genetic disorder who may have found a way to make himself visible.
Peter and Lincoln visit the other universe, where Lincoln wants to uncover the new shape-shifters' agenda and Peter hopes to find a way home with help from Walternate. But the mission goes very wrong when their real identities are discovered.
The Fringe team searches for a young artist with the ability to see death before it happens, which becomes even more crucial after she predicts a mass-casualty event. Meanwhile, Olivia is preoccupied by her own personal warning from one of the Observers.
Olivia and Peter search for a man who can predict people's deaths, and is killing them to spare further pain and misery. The other Astrid shows up in Walter's lab with a very personal mission.
Peter, Walter, and Olivia find themselves trapped in a small Vermont town where residents appear to be afflicted by severe schizophrenia with supernatural origins. Meanwhile, Olivia herself experiences memories of her alternate history with Peter.
After a mental patient appears to witness a murder without being present at the crime scene, Olivia and Peter suspect that he has a telepathic connection to the killers. Walter grows suspicious that Massive Dynamic may have reverse-engineered Cortexiphan.
As David Robert Jones tortures Nina in order to activate Olivia's abilities, Peter realizes that the only way to find her is to enter the mind of the rogue Observer called "September."
A murderer targets couples in love, extracting concentrated pheromones from the men in order to create a personal moment with their significant others. Meanwhile, Peter discovers that the Observer left him a hidden message.
After an airline passenger transforms into a massive creature, Peter recognizes the case as one from his timeline -- but now events are unfolding differently. Olivia defies FBI orders after she is taken off active duty.
Lincoln takes a trip to the other side and finds himself drawn into a case with the other Fringe Division, chasing a very unusual shape-shifter in the hope of finding David Robert Jones.
The mysterious deaths of three people turn out to be connected to a plane crash that killed their doppelgangers on the other side, sending Walter over to team with Lincoln and the other Olivia. Meanwhile, Olivia's investigation into the mole looks to be hopeless.
In the year 2036 two Fringe agents may have finally found someone who can help them free the world from the dystopian rule of the Observers: Dr. Walter Bishop, frozen in amber for more than 20 years.
With the realization that Jones is using former Cortexiphan test subjects in a plan to collapse the two universes, both sides must weigh the cost of shutting down the bridge that joins them together. Olivia reconnects with a man from her childhood.
After an attack using advanced nanites causes victims to spontaneously combust from the inside out, Olivia and Peter hunt for David Robert Jones -- but Walter suspects that an old friend may be the real culprit.
The team searches for Walter, who now stands with William Bell on the cusp of the birth of a brand new universe. The Observer September returns, and a former patient turns out to be more than she appears.
