 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 . Mulder is a believer in the paranormal , and the skeptical Scully has been assigned to debunk his work . In this episode , Mulder and Scully find that a child killer ( Tom Noonan ) who Mulder had helped to apprehend several years earlier had claimed more victims than he had confessed to ; and in the resulting investigation , learn that the killer is now claiming to have killed Mulder 's sister Samantha . 
