Set in Boston, the FBI's Fringe Division started when Special Agent Olivia Dunham enlisted institutionalized Fringe scientist Walter Bishop and his globe-trotting, jack-of-all-trades son, Peter, to help in an investigation of an airline disaster that defied human logic. After the defining case was solved and furthermore revealed to be one of a series of unusual incidents linked together, the unlikely trio - supervised by Special Agent Phillip Broyles and assisted by Astrid Farnsworth - was formed.

As unimaginable events continued to unfold, the Fringe Division's investigations often led them to science and technological corporation Massive Dynamic and its enigmatic chief executive, Nina Sharp. It was revealed that Walter and Massive Dynamic's elusive founder and chairman William Bell once were scientific partners, and had used Olivia as a test subject when she was a child.

Through the Fringe team's investigations, Olivia learned that the root cause of these strange occurrences was due to the existence of a parallel universe. Even more shocking was the discovery that Walter had kidnapped Peter as a child from "over there" after his Peter had died, thus creating an imbalance between the two universes that threatened to tear both worlds apart.

Fringe S4 airs Thursdays at 21:30.