"Folie à Deux" is the nineteenth episode of the fifth season of American science fiction television series The X-Files.

The show centers on FBI special agents Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson) who work on cases linked to the paranormal, called X-Files.

To fix the perceived issues with the monster, the production team gave the film to visual effects editor Laurie Kallsen-George, who digitally altered the footage until it was deemed suitable.

In Oak Brook, Illinois, Gary Lambert (Brian Markinson), a telemarketer at a company called VinylRight, believes that his seemingly normal boss, Greg Pincus (John Apicella), is an insect-like monster that only he can see in its true form.

Walter Skinner (Mitch Pileggi) orders Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson) to go to Chicago to do a threat assessment of a taped manifesto that mentions VinylRight, which has seen a violent incident at its Kansas City offices.

Meanwhile, Scully calls Mulder to tell him that she found the phrase "hiding in the light" in a 1992 case from Lakeland, Florida, that involved similar accusations of hidden monsters.

He returns to the VinylRight office and unwittingly walks into the middle of a hostage situation, being held captive by Lambert along with Pincus and the other employees.

As Scully arrives on the scene, Lambert divides his hostages into "real people" and "monsters", claiming that Pincus has turned several employees into zombies.

When Lambert demands a camera to broadcast his warning, Scully arranges for a SWAT officer disguised as a cameraman to be sent in the building.

Mulder tells Scully that he saw the monster doing something to the back of his victim's head, begging her to look for similar evidence on Backus's body.

[1] "Folie à Deux" was written by Vince Gilligan, who was inspired by the idea that "there's a monster around that only you can see—the clinical definition of madness".

[2] The title is a reference to Folie à deux (French for "madness of two"), a psychiatric syndrome in which symptoms of a delusional belief and sometimes hallucinations are transmitted from one individual to another.

"[2] Director Kim Manners too thought the costume was ridiculous and sardonically told people that the episode would ruin his career.

"[2] Initially, the producers wanted to set the Gretchen Starns' house scenes at a tract home, to recall the "pastel community" featured in movies like Edward Scissorhands (1990) and The Truman Show (1998).

[6] The elaborate sequence in which the insect creature crawls across a ceiling was made by erecting an upside-down house set on a soundstage.

[2] The scenes featuring an armored car smashing through the wall of VinylRight were also filmed on a specially constructed soundstage.

[7] "Folie à Deux" originally aired on the Fox network on May 10, 1998, and was first broadcast in the United Kingdom on BBC One on March 10, 1999.

[12] Robert Shearman and Lars Pearson, in their book Wanting to Believe: A Critical Guide to The X-Files, Millennium & The Lone Gunmen, rated the episode four stars out of five.

The two compared the entry to the previous fifth season installment "Bad Blood", calling "Folie à Deux" its "logical extension".