The episode featuring a killer who causes spontaneous human combustion is “Fire.”
“Fire” is the twelfth episode of the first season of The X-Files, although its production code identifies it as 1X11. It first aired on December 17, 1993, and was written by series creator Chris Carter and directed by Larry Shaw. The story opens in Bosham, England, where a wealthy man suddenly catches fire and burns to death in front of his home.
The case becomes linked to a serial arsonist targeting members of the British aristocracy. Fox Mulder and Dana Scully investigate after Mulder’s former Oxford acquaintance, Metropolitan Police investigator Phoebe Green, asks for help protecting another potential victim. The suspected killer, Cecil L’Ively, appears able to ignite himself and project flames, making the episode a supernatural “monster-of-the-week” story rather than part of the show’s central alien mythology.
“Fire” is sometimes confused with “Firewalker,” another first-season episode, but “Firewalker” concerns a deadly organism discovered on a volcanic expedition. The title “The Fire Within” is not the relevant X-Files episode.