Underneath (The X-Files)

"Underneath" is the twelfth episode of the ninth season of the American science fiction television series The X-Files. The episode first aired in the United States on March 31, 2002, on the Fox network. The episode was written and directed by executive producer John Shiban. The episode is a "monster-of-the-week" episode, a stand-alone plot which is unconnected to the mythology, or overarching fictional history, of The X-Files. The episode earned a Nielsen rating of 4.4 and was viewed by 4.64 million households and 7.3 million viewers. It received mixed reviews from critics.

The show centers on FBI special agents who work on cases linked to the paranormal, called X-Files; this season focuses on the investigations of John Doggett (Robert Patrick), Monica Reyes (Annabeth Gish), and Dana Scully (Gillian Anderson). In this episode, Doggett is determined to find an error in the DNA evidence that freed the convicted Robert Fassl, the "Screwdriver Killer", whom he nearly caught in the act 13 years earlier. In the end, it is revealed that Fassl has a mental condition that splits him into two parts: the religious innocent and the vengeful killer.

"Underneath" marked the directorial debut of Shiban, who had been a writer for the series for several seasons. Reportedly, the episode contained "so many problems" that the Fox executives nearly refused to allow the finished product to air. At the last minute, however, they relented, and allowed the episode to be aired later on in the season, several weeks after its intended air date. Shiban originally wanted to film the sewer scenes in Los Angeles' actual sewer system, but due to the events of September 11, a sewer mock-up was built on Stage 11 at the Fox studios.