Six years have passed since the death of “The X-Files,” the long-lived mutant offspring of “Twin Peaks” and “Unsolved Mysteries.” Now, older if not wiser, Scully and Mulder return for “The X-Files: I Want to Believe,” which on its own merits is little better than a cable thriller with ponderous interludes. It’s technically true that the new film is accessible to the uninitiated, but the mediocre material may only interest those with prior emotional or paranormal investment.

An earlier spin-off feature, subtitled “Fight the Future” and released at the show’s height in 1998, roiled with the show’s febrile matrix of extraterrestrial intrigue. In “I Want to Believe,” the series’s creator, Chris Carter, who directs from a script he wrote with a longtime show scenarist, Frank Spotnitz, puts his faith in a stand-alone story. It’s essentially a Frankensteinian B-horror premise that gives its star duo the excuse to muse on whether they still believe.

Neither agent is officially in the spookiness racket anymore. Dana Scully (Gillian Anderson) is putting her medical degree to work as a pediatric surgeon at a Catholic hospital. The FBI, stymied by the case of an agent’s disappearance, enlists her to coax out Fox Mulder (David Duchovny). He’s still in hiding from the military, as per the un-summarizable state of affairs left by the series finale way back when.

The case, introduced with a parallel-action opening belying years of finding fresh ways to begin episodes, is standard-issue: grisly murders (limbs and bodies found buried in the snow), tracked by an unsavory psychic (Billy Connolly as Father Joe, a pedophile ex-priest). Against the wintry backdrop of a whited-out countryside and occasionally Vancouver, Mulder and Scully trade references, regrets, and avowals, though she’s also hesitant and preoccupied with a dying patient. Aggressively useless FBI investigators (headed up by Amanda Peet and Xzibit) festoon the proceedings with police manpower and insinuations.

The play of skepticism and headlong curiosity that first gave the pair their appeal is deadened by Scully’s unease about the whole endeavor, and its romantic corollary is tossed off with an almost amusing casualness. Deprived of the paranoid pleasures of the show’s “mythology,” the writers serve up some Slavs engaged in involuntary body-part acquisition and stir up a vague Catholic menace via Father Joe and a hard-nosed priest administrator at Scully’s hospital. (Contemporary markers include a dig at President Bush and a regrettable clue involving the Massachusetts marriage certificate of two male villains.)

Maybe this would be okay television, but like an unflattering close-up, this particular X-file loses its mystique when it is blown up to feature length. Falling short of the TV show’s tangled yarns, which at least made incredible occurrences entertaining, the movie has saggy suspense and lazy moments, such as Scully’s Google-based detective work and Mulder’s survival of multiple car rolls. And the self-serious signposting of themes and sentiments (“Don’t Give Up”) that we accept in television will here probably resonate only with people who come already attached to the characters.

The conclusion of a typical episode — the part where you politely look away as things are wrapped up or fluffed for the next installment — is tiringly elongated over the film’s last third. At least the show’s stars settle into their signature characters without treating them as albatrosses round their necks, even though Ms. Anderson has recently been seen introducing Jane Austen adaptations for “Masterpiece Theater” and Mr. Duchovny skirt-chasing in Showtime’s worst-titled show ever, “Californication.”

Almost exactly one year ago another influential Fox hit, “The Simpsons,” successfully jumped to the big screen after years of anticipation. But this summer the already weak “X-Files” movie is upstaged by the weirdly fervid national cult surrounding a bat-man. Maybe the diversion is a blessing in disguise, because the release of “I Want to Believe” is a little like the attempt of its body-snatching baddies to cheat death.