 " 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 April 27 , 1997 . 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 . 
