It's a mystery worthy of Fox Mulder's scrutiny: Why make a theatrical motion picture of that iconic '90s TV series "The X-Files" six years after the last original episode aired? Nobody's doing "Seinfeld: The Movie," "Friends Forever" or "Walker, Texas Ranger, Rides Again."

But "The X-Files" is science fiction, and as we know from Trekkies or Trekkers or whatever the FC (fannishly correct) term is these days, sci-fi love never dies. And this past February at the WonderCon convention in San Francisco, those true believers known as X-Philes were sending X's — as in kisses — to the movie's producers and stars on a panel promoting "The X-Files: I Want to Believe," opening Friday.

From the screams and squeals captured in a featurette on the new DVD episode-set "The X-Files: Revelations," you'd think Elvis was in the building.

"We wanted this movie to work for and be loved by fans of the show," says former series executive producer Frank Spotnitz, who wrote and produced the movie with "X-Files" creator Chris Carter. "But if it only works for them," he frets, "it's not a success."

It does make your mind boldly go to "Star Trek: The Motion Picture" (1979), released 10 years after the TV series ended and five years after the Saturday-morning animated spinoff. Like that first "Star Trek" movie, the 1998 film "The X-Files" got a middling reception at best. And if this second film turns things around like the critical and commercial hit "Star Trek II: The Wrath of Khan," stars David Duchovny and Gillian Anderson might just wind up this generation's William Shatner and Leonard Nimoy (or, perhaps, Nichelle Nichols).

The movie takes place six years after the events of the series finale, which, bizarrely, echoed the final "Seinfeld": a trial, with recurring characters from the series showing up to testify and yada yada yada. In the end, FBI agent Mulder (Duchovny) — investigator of aliens, monsters and other things that go bump on the screen — escaped his kangaroo court with the help of, among others, his FBI superior, Walter Skinner (Mitch Pileggi, who appears in the film). Mulder goes on the run with his no-longer-skeptical partner, Dana Scully (Anderson). We last see them together in a hotel room, pondering the future and what to believe in.

Unlike "Lost," which has a similarly supernatural back-story mythos, the final "X-Files" seasons were all dangling and muddled loose ends. If the producers ever had a planned-ahead revelation, answer or overall point, it had gotten, well, lost.

"The show clearly didn't unfold the way anybody anticipated," Spotnitz says. "It doesn't have the great circularity of a novel. It changed gears in ways we hadn't anticipated. I understand why a lot of fans weren't as emotionally invested in those last two years as in the rest."

The harsh realm of reality didn't help. "I have come to believe the show ended when it did because of the post-9/11 mood of the country," Spotnitz says. "I remember the Sunday New York Times Magazine a couple weeks after 9/11 having a list of things that were 'in' and 'out,' and 'The X-Files,' it said, was out. And I thought, 'Why would we be out?' And when we came back on TV after that, the audience just didn't show up for Season Nine. People felt it was an anti-government show, and out of step with the mood of the country."

Now? "My sense is we're now in a 'post-post-9/11' frame of mind."