No need for a crash refresher course on “X-Files” mythology before going to see Chris Carter’s feature resuscitation of his celebrated series. Scully and Mulder are back, but in “I Want to Believe,” the labyrinthine back-story spun for them over the show’s nine-year span is largely jettisoned in favor of a stand-alone plot, not unlike the “Kolchak”-inspired episodes that stood apart from the “vast conspiracy” ones in the series. In that respect it’s different from the 1998 feature, which fit snugly into, and advanced, the program’s space-invasion narrative thread. But after all, when the series ended in 2002, it not only pretty much tied its convoluted alien scenario up in a trial that’s briefly alluded to here, but made it clear that the next chapter in that saga wasn’t due until 2012 (according to the Mayan calendar). So even though this new chapter has been six years coming—delayed by a legal brouhaha, among other things—it’s still four years too early for that.

Unhappily, the wait apparently hasn’t been long enough for series honchos Carter and Frank Spotnitz to come up with a script worthy of a big-screen revival. That’s not to say that the more intimate material between David Duchovny’s still-credulous Mulder and Gillian Anderson’s still-skeptical Scully isn’t acceptable. For fans who retain a warm spot in their hearts for the actors and the characters they created, it will be a welcome experience just to see them together again, even if they are an awfully gloomy pair after more than half a decade away from their FBI office. (He’s been hiding out as a recluse after escaping the series finale’s attempt to discredit and silence him permanently, while she’s taken a position as a surgeon at a hospital that brings her apparently hopeless cases—like that of a young boy with apparently inoperable brain cancer, who undoubtedly reminds her of the son she and Mulder have up for adoption.) The chemistry between the two is still there, and it’s a nice surprise when an old ally from the show shows up to assist them at a crucial point, too. (I won’t say who, not even in the cast listing.)

But the case that draws both of them out of retirement into the fray proves a weak thread on which to hang their reunion. It’s basically a missing-persons episode involving a kidnapped FBI agent. But of course it moves into paranormal terrain with the presence of a psychic, a defrocked priest named Father Joe Crissman (Billy Connolly), whose visions of the crime lead our heroes into serial-killer territory with a particularly nasty twist. (To say more would constitute the sort of spoiler that goes beyond reviewing propriety.) The ecclesiastical subplot links things up with Scully’s hospital dilemma, since she’s working at a Catholic facility where nuns are constantly fluttering about, and more importantly the priest-administrator (Adam Godley) objects to continuing treatment for patients, like Scully’s boy, for whom there is no effective therapy, and Scully turns to experimental stem-cell work as a last resort. (Come to think of it, there’s a lot of brooding Catholic gloom and guilt here. What’s up with that?)

Father Joe’s involvement reasserts the old Mulder-Scully duality between belief and incredulity—he’s willing to accept the possibility that the man is a true visionary, while she dismisses him as a despicable fraud—and the point is reinforced by the presence of a second FBI team: Dakota Whitney (Amanda Peet), who believes in Mulder, and her partner Mosley Drummy (Alvin “Xzibit” Joiner), who thinks he’s a nutbag). That’s fine. What isn’t is that the malevolent scheme the investigation leads to isn’t just absurd in the fashion of so many rote post-“Se7en” imitators, but thoroughly unpleasant, with visuals reminiscent of the sort of grisly excesses one finds in the “Hostel” franchise. It’s clear that the makers are aiming for the pleasurably creepy tone of some of the series’ best episodes—think of “Home,” for example, one of the most unsettling hours ever broadcast—but they fail. The result is ridiculously grotesque rather than truly frightening, and it misses almost completely the sense of humor of the original. (The presence of a bunch of babbling Russians in the mix is a particular mistake. It turns out the movie has plenty of aliens, just of the wrong kind. In fact, the creepiest-funniest moment in the whole movie has nothing to do with the plot; it’s a throwaway in a hallway when we see pictures of George W. Bush and J. Edgar Hoover side-by-side.) Adding to the problem is the fact that in the final analysis the elements of the plot don’t compute. To cite one point: a slew of bodies eventually turn up, yet there’s no indication that the authorities had earlier been even vaguely aware that people had been going missing. And linkages involving medical identification bracelets, animal tranquilizers, an organ-delivery firm and a long-ago case of child abuse will be difficult to discern, even for the most attentive viewers.

“I Want To Believe” has an effectively gloomy look, thanks to cinematographer Bill Roe, who also uses the snowy Vancouver locales to give the picture palpably chilly feel. Mark Snow’s score, reviving the old show theme, is also effective. And while Duchovny and Scully are the main strengths of the cast, Peet is actually quite good as the agent heading the investigation. (Xzibit, on the other hand, is terrible. He definitely needs more acting lessons before he gives up his rap job.) Connolly goes the overwrought route fairly effectively, but the rest of the supporting cast—especially those playing the Russkis—come across as borderline comic rather than terrifying.

As a fan of “The X-Files”—I watched the program from the very first episode through the finale, even the mediocre last season—I really wanted to like “I Want To Believe.” Unfortunately, even those who retain great affection for Mulder and Scully are likely to find this reunion at once disappointingly routine and overly glum and grim, reminiscent of some of the series’ weaker episodes.