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 team as they investigate a shapeshifter case that is very personal to him. 

The victims of a highly intelligent serial killer are piling up. 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. 

The Fringe team investigates when the bodies of two 12-year-olds are found in an advanced state of decomposition just hours after bullying another boy; Walter grows more and more distracted by hallucinations. 

Walter's experiments with a new drug result in a dangerous force; Olivia and Walter examine old case files in New York; Walter struggles with life outside the lab. 

The translucent shapeshifters return. As the threat and circumstances intensify, a former Massive Dynamic scientist named Malcolm Truss enters the fold. Meanwhile, Nina Sharp pays Walter a visit in the lab. 

The team must find the pattern in tragic time-loop anomalies. As the investigation unfolds, an electrical engineer and his wife -- a professor of theoretical physics -- enter the picture. 

The team investigate a man's mysterious death and are led to a culprit of the invisible variety. Meanwhile, Olivia starts suffering from a spate of migraines. 

Peter asks Olivia's help in going to the Other Side. Walternate's aid is needed when Walter refuses to help Peter employ the Machine to return to his own timeline. Meanwhile, the threat of the prototype shapeshifters grows and an old enemy returns. 

Peter reaches an uneasy alliance with the Other Side's Fringe Division and helps them once they capture the mastermind behind the shapeshifters. Meanwhile, the Other Side's Elizabeth Bishop comes to see Walter Bishop to ask his help. 

The team discover a girl who can see the future and she warns that her premonitions always come true. Olivia and the others must find the source of her newest vision, which shows a courthouse collapsing and killing hundreds of people. 

While the Division deals with a murderer who uses a poison that can't possibly exist, Agents Farnsworth and Dunham from the Other Side come to pay them a visit. 

Peter, Olivia and Walter are investigating a plane crash near the small Vermont town of Westfield. They soon discover that they are trapped in the town and surrounded by people who are going insane. 

When Olivia and the team investigate a mental patient, he describes a real-life murder. Olivia begins to be overwhelmed by the memories of Peter's Olivia, and they must decide if they wish to give in and accept Olivia's new persona 

As David Robert Jones tortures a captive Nina in an effort to activate Olivia's cortexiphan-enhanced abilities, the dying Observer visits Peter and Walter at their lab and Peter must merge minds with him to learn his secrets. 

After recent emotional events and shocking revelations, Peter considers his options; a killer targets love. 

In a plot-thickening episode, the familiar becomes the unfamiliar when the team take on a case that strikes a chord with Peter. 

On the other side, Lincoln joins forces with his alternate and Bolivia as they look for a vigilante with a connection to the shapeshifters. 

While Walter takes a trip to the alternate universe to assist in an investigation that has ties to both worlds, David Robert Jones reveals his next target. 

The Observers and the team engage in a battle in the year 2036. 

The teams on both sides fight on for a common cause, and there are shocking developments tied to the Cortexiphan children. 

Walter is forced to revisit his painful past when a fringe event causes people to spontaneously combust; the team faces off with David Robert Jones. 

All bets are off as the team tries to prevent a catastrophic event that threatens the lives of everyone.