"Zero Sum" is the twenty-first episode of the fourth season of the American science fiction television series The X-Files. It premiered on the Fox network on . It was directed by Kim Manners, and written by Frank Spotnitz and Howard Gordon. "Zero Sum" included appearances by William B. Davis, Laurie Holden and Morris Panych. The episode helped to explore the overarching mythology, or fictional history of The X-Files. "Zero Sum" earned a Nielsen household rating of 11.7, being watched by 18.6 million people in its initial broadcast. The episode has received mixed to positive responses from critics. 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. In the episode, a case Mulder is asked to investigate is covertly covered up by the agents' boss Walter Skinner (Mitch Pileggi), who has made a sinister bargain with The Smoking Man (Davis). The decision to center another episode around Skinner—following the third season episode "Avatar"—was made when Gillian Anderson took a week off to shoot the film The Mighty. Rather than have Duchovny carry the additional workload, the writing staff decided to focus the episode on supporting cast members. "Zero Sum" was the final appearance in the series by Morris Panych. In addition, the episode saw the return of the virus-carrying bees from the season-opener "Herrenvolk" and would later return in the 1998 feature film. Plot At a postal routing center in Vienna, Virginia, a woman is killed by a swarm of bees in the bathroom while taking a cigarette break. Assistant Director Walter Skinner (Mitch Pileggi), working for The Smoking Man (William B. Davis), covers up the death by deleting the file on the case from agent Fox Mulder's (David Duchovny) computer, cleaning up all the evidence at the scene, burning the woman's body in an incinerator and replacing the police's blood sample for the case by impersonating Mulder. As he leaves the police station, Skinner is chased down by Detective Ray Thomas, who believes he is Mulder. Skinner tells him there is nothing on the case warranting his involvement, and leaves. Shortly after Skinner arrives home he is met by Mulder, who tells him about the case and the fact that someone is going to great lengths to cover it up. Mulder reveals that Detective Thomas was killed, which shocks Skinner. Mulder tells Skinner