"Dreamland" is the title of both the fourth and fifth episodes of the sixth season of the science fiction television series The X-Files. Part one first aired on November 29, and part two aired on December 6, 1998, on Fox in the United States and Canada. The episode was written by Vince Gilligan, John Shiban, and Frank Spotnitz, and directed by Rob Bowman ("Dreamland") and Michael Watkins ("Dreamland II"). Although dealing with a member of the Men in Black, the episode is largely unconnected to the mythology of The X-Files, and is a "Monster-of-the-Week" story. Part one of "Dreamland" earned a Nielsen household rating of 10.1, being watched by 17.48 million people in its initial broadcast; Part two received a rating of 10 and was watched by 17.01 million people. The episode received mostly mixed reviews from critics, with many reviews critical of the episode's reliance on humor.

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. Mulder is a believer in the paranormal, while the skeptical Scully has been assigned to debunk his work. In the episode, Mulder and Scully visit Area 51. But when the agents witness the flight of a mysterious craft, Mulder and a member of the Men in Black switch bodies, unbeknownst to the others. In part two, Scully begins to suspect that her partner's strange behavior is more than it appears to be, while Mulder fights to return his life to normal before it is too late.

"Dreamland" was the only non-mythology story to be spread across two episodes. Originally, the writers for the episode contacted Garry Shandling to play the part of Fletcher but he was unavailable for filming. Michael McKean was then cast in his place. The episode is notable for a scene featuring Mulder-as-Fletcher encountering his reflection and proceeding to do the dance from the 1933 Marx Brothers comedy film Duck Soup. The scene required Duchovny and McKean to synchronize their movements perfectly. Dreamland is one of the names used for the Area 51 facility.

Fox Mulder (David Duchovny) and Dana Scully (Gillian Anderson) visit the Area 51 installation in Nevada after receiving a tip from an inside source concerning alien spacecraft. As they drive on the highway, the agents are surrounded by jeeps carrying soldiers led by a Man in Black named Morris Fletcher (Michael McKean). A rumbling noise is heard as a mysterious craft flies overhead and a bright light from the object passes over them. Mulder and Fletcher find that their minds have been swapped into each other's bodies, but nobody else present is aware of this. Fletcher and Scully depart, watched by the soldiers.

Mulder is driven back to Area 51 along with two other Men in Black, Howard Grodin and Jeff Smoodge. After an angry telephone call from Fletcher's wife, Joanne, Mulder—still inside Fletcher's body—goes to Fletcher's home. Instead of sleeping in the bedroom, he decides to sleep downstairs in an easy chair and watch pornography. When he is awoken by Joanne, he mumbles Scully's name. While changing his clothes, Mulder receives a call from Smoodge, who explains that the military has surveyed the wreckage of the craft from the night before, finding one of the human pilots fused into a rock but still alive. Another soldier, Captain Robert McDonough, had switched bodies with an elderly Hopi woman, Lana Chee, as evident from McDonough's aberrant behavior.

Mulder calls Scully and tries to explain that he's the real Mulder. Scully does not believe him and asks Fletcher—in Mulder's body—to pick up and listen to the conversation. Fletcher decides they should immediately report this incident, which further confuses Scully. She goes to Mulder's apartment (where Fletcher is having a tryst with Assistant Director Kersh's assistant) and tells him that they traced the call to a payphone near Area 51; she suspects it is Mulder's informant. Fletcher is indifferent to this news and Scully yells at him, sensing that his behavior is very strange and his lack of concern for the X-files is completely out of character.

Deciding to investigate on her own, Scully drives through the desert towards Area 51. She stops at a burned out gas station and finds a penny and a dime fused together. Later, when Scully arrives at Fletcher's house, Mulder tries desperately to convince her of his identity. Scully remains skeptical and thinks that any information Mulder describes could be obtained in other ways. Mulder tells Scully that he will provide proof, but Fletcher, having eavesdropped on the conversation, calls his old office posing as Mulder to inform them of the source of the leak—Mulder posing as Fletcher. Military police arrive at the appointed hour, and Mulder is dragged away, desperately pleading with Scully. Mulder asks if he would turn in an informant to the authorities in this manner as he is carried away, and Scully begins to realize that he may be telling the truth.

As Mulder—in Fletcher's body—is dragged away by the soldiers, Scully begins to question whether his story about the body-swap was true. Fletcher—in Mulder's body—approaches Scully and apologizes for telling Kersh; she feigns acceptance of it. After being reprimanded, Fletcher arranges a dinner date with Scully at Mulder's apartment. Meanwhile, Mulder is confined in a cell next to the mind-switched Lana Chee. Military police take Mulder to a meeting with General Wegman, Grodin and Smoodge, who believe that Mulder-as-Fletcher was trying to defraud the FBI, not help them. Mulder bluffs his way through the meeting, claiming that the real proof is safe and that he did not tell them about the scheme because he did not know if he could trust his colleagues, thinking any one of them could be the source of the leak. Mulder returns to Fletcher's house and tells Joanne of his true identity, but she thinks her husband must be undergoing a mid-life crisis.