An Observer makes contact with the Fringe Team to help rectify a mistake. Meanwhile, Walter befriends Roscoe Joyce, keyboardist for Walter's favorite 1970s band, "Violet Sedan Chair." Walter will learn the real reason the band broke up, and realize that the lives of these two men, who have never met before, overlap in strange and unexpected ways.
When the Fringe Team visits Massive Dynamic's assembly of doomsday device, Walter becomes greatly concerned for Peter's well-being, so he turns to Nina for added brain power to understand the relationship between Peter and the super-weapon. Meanwhile, the discovery of a dead body triggers an intense investigation.
When a scientist falls dead after ingesting a lethal cloud of blue powder and his bones disintegrate in his body, the CDC suspects a biological attack. As the Fringe Team investigates further, a subject from Walter's past with ties to Olivia reluctantly agrees to assist with the intense case.
As the story shifts back to life "over there," the absence of Colonel Broyles shifts the dynamic of the Fringe Team as they investigate a bioterrorist armed with an insect that has a taste for human flesh. Meanwhile, alternate Olivia is reunited with her beau, and Walternate remains determined to save his world but discovers there are certain lines he will not cross.
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.
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.
As the victims of a highly intelligent serial killer "Over There" stack up, the Fringe Division "Over Here" is asked to assist. As the two sides tangle and innocent people remain at risk, the suspect's doppelganger "Over Here," a professor who teaches Forensic Psychology, is brought into the fold and forces the team to consider the notion of sending civilians to the other side.
When two 12-year-old kids bully another boy, they are found dead and in just hours after dying their bodies are shockingly in an advanced state of decomposition. As the Fringe team investigates the mysterious case, they uncover an amorphous figure claiming more victims. Meanwhile, Walter becomes increasingly distracted by his hallucinations.
Olivia uncovers a dangerous force that may be connected to Walter's past experiments with the drug Cortexiphan. Their investigation forces Walter to leave the lab for the first time in years as he and Olivia head to Massive Dynamic in New York to examine old case files. Walter struggles with life outside the lab as he comes face-to-face with the demons of his past.
In the aftermath of recent shocking events, things really escalate when the translucent shapeshifters return. As the threat and circumstances intensify, former Massive Dynamic scientist Malcolm Truss enters the fold. Meanwhile, Nina Sharp pays Walter a visit in the lab.
The Fringe team must find the pattern between tragic time loop anomalies. As the investigation unfolds, Raymond, an electrical engineer, and his wife Kate, a professor of theoretical physics, enter the picture.
Murders occur in which the victims are initially thought to be albinos, but when the body count reaches four, it seems evident something else is happening and gets the attention of Fringe Division. Blood at one such scene is traced to a strange, pale baby hypersensitive to light who supposedly died 4 days after his birth in 1989. But U-gene (for Unknown) Bryant almost certainly survived and came to the attention of the body that later became Massive Dynamic, where he became the subject of intensive experiments. Walter demonstrates that UV light can reveal an otherwise...