Mulder is an Oxford-trained psychologist with a wry sense of humor and a fascination with the unknown. The key to his psyche is his childhood trauma; he watched as his sister was abducted by aliens. His creed: ““The truth is out there.’’ Scully graduated from medical school and then joined the FBI. She was assigned to the X-Files, largely to monitor Mulder (colleagues call him ““Spooky’’).

Although the fans have all kinds of theories about why the show is popular, creator Chris Carter says he just wanted to scare people. Among his inspirations were Mary Shelley’s novel ““Frankenstein’’ and the 1970s TV show ““The Night Stalker,’’ about a reporter who tracked down vampires. Some of the most compelling episodes have been written by Glen Morgan and James Wong, who grew up together near San Diego, Calif. Before ““X-Files,’’ they wrote for ““The Commish’’; now, they say they draw on such influences as ““Rosemary’s Baby.’’ And the show’s feel, from camera work to lights to music, is at least as freaky as its plots.

““X-Files’’ attracts both male and fe-male fans, largely because Mulder and Scully are appealing role models. They are intimate friends – but never sexually intimate. ““What’s more interesting is someone who can meet you in a conversation or a debate, that exchange of ideas,’’ says Carter. Both leads are serious people, and, as it happens, very good-looking people. In the online discussion groups, male contributors frequently refer to themselves as proud members of the GATB, the Gillian Anderson Testosterone Brigade. Mulder has his admirers, too, the DDEB, David Duchovny Estrogen Brigade.

Like other shows that have attracted a cult following, the X-philes discussions – especially the Usenet group, alt.tv.x-files – are filled with a multitude of details. ““Even the most esoteric question can usually be answered in the newsgroup,’’ says Cliff Chen, a graduate student at the University of Pennsylvania who edits a weekly episode update on the Net. ““For any huge X-phile, knowing things like David Duchovny’s birthdate and Fox Mulder’s astrological sign is a thrill unto itself.’’ Carter monitors reactions online and the writers have often included fan references. For example, an airplane-passenger manifest featured names of frequent discussion-group participants (and a crucial clue).

As ““X-Files’’ becomes more widely known, some fans worry that it will turn too mainstream. Morgan and Wong are leaving to create their own show, which they describe as a World War II movie in space. Carter promises to keep it creepy. ““The material,’’ he says, ““is out there.’’ Sounds like a case for ““The X-Files.''