Fringe/Recap/S01 E13

Season 1, Episode 13:

The Transformation
During a plane flight, a man undergoes a transformation into a kind of monstrous creature who kills everyone aboard. When the Fringe team investigates, Olivia recognises the man from John Scott's memories and tries to track down another individual who she saw meeting with John and the first victim. The team learns that a weapons dealer is planning to sell the formula that caused the transformation, and from another trip in the tank, Olivia learns that John was part of an undercover NSA taskforce trying to bring the weapons dealer down. Olivia and Peter pretend to be sellers in order to lure the weapons dealer into the open and arrest him. Satisfied that John wasn't a traitor after all, Olivia goes in the tank to see John one last time, before his memories fade altogether.