When a specific group of guests at a wedding in Brookline, MA, suffocate from the inside out, the upsetting and disconcerting crime scene has the Fringe team investigating how the toxin was delivered and targeted. As evidence is taken into the lab, it's discovered that the wedding ceremony was a testing ground for a weaponized science experiment. Adding to the intense lab investigation, Walter uncovers an alarming formula that reveals a link to a branch on the Bishop family tree. With the escalating threat of more deadly attacks and unexpected familial ties to the case, the Fringe team stops at nothing to prevent further catastrophic events.

When a man enters an office in Boston for a meeting, he becomes disoriented and collapses, only to have his veins erupt and spray blood everywhere. Shortly after Olivia and Peter arrive at the scene to investigate the bizarre case, it's discovered that a lethal virus with deep ties to something very valuable is spreading, so the CDC moves in to quarantine the building and eradicate the pathogens. Isolated inside with the rest of the exposed office employees, Peter and Olivia face life-threatening circumstances, sending Walter, Broyles and Astrid on a frantic search for a solution to save them.

Following an unexplained attack involving disfigured humans, the Fringe team visits Edina City, a small town in upstate New York to uncover leads surrounding the bizarre case. When it's determined that these disfigured people have managed to hide themselves for a while and they'll do just about anything to keep it that way, the investigation takes an unexpected turn.

A teenage girl pronounced brain dead wakes up in the middle of the operation to remove her organs... speaking numbers in Russian. When the Navy confirms that the numbers are nuclear launch codes and a missing officer named Rusk knows them, the search is on for the missing man. '

During the investigation of strange circumstances surrounding brain surgeries at a mental hospital, Olivia finds a reason to believe the leader of the shape-shifters may have something to do with it. Leonard Nimoy guest stars as William Bell

After a cargo ship runs aground, the shoreline becomes littered with bodies hosting a giant squid-like creature. The Fringe team descends upon the scene to examine the bodies and discovers that the mysterious organisms are actually giant parasites. As the investigation unfolds, Walter heads back to the lab while Olivia, Peter and Broyles race against time when they realize the case has ties to a threatening organization.