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Originally telecast February 14, 1999, "One Son" was the conclusion of a two-part X-Files drama that began with the previous week's "Two Fathers." In this pivotal episode, Mulder finally learns the truth about the government conspiracy responsible for the disappearance of his sister. Players essential to the action include celebrated -- and mysterious -- alien abductee Cassandra Spender (Veronica Cartwright), Spender's FBI-agent son, Jeffrey (Chris Owens), apparently duplicitous Agent Diana Fowley (Mimi Rogers), and Mulder and Scully's longtime bête noire, the Cigarette Smoking Man (William B. Davis). "One Son" was written by series producers Chris Carter and Frank Spotnitz.