The mythology of , sometimes referred to as its "mytharc" by the show's staff and fans, follows the quest of FBI Special Agents Fox Mulder (David Duchovny), a believer in supernatural phenomena, and Dana Scully (Gillian Anderson), his skeptical partner. Their boss, FBI Assistant Director Walter Skinner, was also often involved. Beginning with season 8, another skeptic named John Doggett, and Monica Reyes, a believer like Mulder, were also introduced. The overarching story, which spans events as early as the 1940s, is built around a government conspiracy to hide the truth about alien existence and their doomsday plan. Not all episodes advanced the mythology plot, but those that did were often set up by Mulder or Scully via an opening monologue.
Most mythological elements in The X-Files relate to extraterrestrial beings, referred to by the writers as "", whose primary goal is to colonize Earth. Late in the series, this was revealed to have been planned for the year 2012.