Some "X-Files" episodes are scary because of the monsters Mulder and Scully encounter, and some because of the psychological terror they induce. The Season 5 episode "Folie à Deux” brilliantly combines both elements, portraying one man's descent into insanity paired with a pretty disturbing monster-of-the-week.
Mulder's sanity is questioned time and time again in "The X-Files," but his state of mind is never as compromised as it is in "Folie à Deux." In this episode, Mulder travels to Illinois to investigate Gary Lambert (Brian Markinson), a telemarketer who believes his boss is a giant insect monster that only he can see. Mulder initially believes the case is a waste of time and tells Scully he will take care of it on his own, only to get wrapped up in the man's psychosis. After being held hostage by Lambert at his office, Mulder begins to see what Lambert sees — his monstrous boss, as well as the other employees he has turned into zombies.
When Scully comes to town she tells Mulder he's experiencing "folie à deux," or "a madness shared by two," despite the fact that Mulder has found evidence that Lambert's boss is up to no good. Like the best — and most disturbing — episodes of "The X-Files," neither Mulder nor Scully find a satisfying explanation for what they experience. While Scully defends Mulder against Skinner and temporarily sees what he sees, the truth never really becomes clear. This uncertainty, of course, is one of the things that makes the episode so terrifying.
