The mythology of The X-Files, sometimes referred to as its "mytharc" by the show's staff and fans, follows the quest of FBI Special Agents Fox Mulder, a believer in supernatural phenomena, and Dana Scully, 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.

"William" is the sixteenth episode of the ninth season of the American science fiction television series The X-Files, which originally aired on the Fox network on April 28, 2002. The teleplay of the episode was written by series creator Chris Carter, from a story by former series star David Duchovny, Carter, and executive producer Frank Spotnitz; the entry was directed by Duchovny. "William" helps to explore the series' overarching mythology. The episode received a Nielsen household rating of 5.8, being watched by 6.1 million households and 9.3 million viewers upon its initial broadcast. It received mixed reviews from television critics, many of whom were unhappy with the episode's conclusion.