There is, perhaps, nothing more disheartening than a movie that opens in such a truly riveting manner, with the clear-cut assurance that what is to follow will be worth the wait, only to take a 180-degree turn toward sloppy writing, meteor-sized plot holes, and an almost complete disregard for characters and relationships that we have just begun to get to know and care about. "Dreamcatcher" is, indeed, such a film, but, fortunately, it is also so ambitious and the first hour so freakishly intriguing and well-crafted that the derailed second half can't completely bring it down. It does come close.

Based on the sprawling 2001 novel by Stephen King, "Dreamcatcher" begins by introducing the viewer to four 30-year-old friends, tight-knit since childhood and all linked by the gift of clairvoyance, given to them years ago by a mentally retarded peer named Duddits (Andrew Robb). Henry (Thomas Jane) is a depressed psychiatrist who often gets himself into trouble by being able to read his patients' minds. Beaver (Jason Lee) is an orally-fixated jokester who can sense what the future holds. Pete (Timothy Olyphant) is a car salesman, unlucky in love and in denial of his alcoholism, who can sense where things are no matter how lost. And Jonesy (Damian Lewis) is an English professor who experiences a ghastly apparition of Duddits before nearly fatally getting hit by a car.

Six months later, Henry, Beaver, Pete and Jonesy join together for their annual weekend at a secluded cabin in the snowy Maine forest. Hoping to peacefully hang out and maybe do some hunting, their trip suddenly takes a turn for the worst with a series of strange occurrences. In an endless stream, the wildlife passes by their cabin, desperately running away from something. They take in a disoriented stranger claiming to be lost who has a terrible case of indigestion and gas. And a military helicopter passes overhead with news of a widespread quarantine in effect for the area. Before long, the foursome come to discover that they are in the midst of a deadly, body-swapping, virus-infecting alien invasion. Back at the military post, Colonel Abraham Curtis (Morgan Freeman), a veteran slowly unraveling from his tireless 30-year alien hunt, plans to wipe out the entire infected area before one of the aliens is unleashed onto the rest of the world.

Written and directed by Lawrence Kasdan (1999's "Mumford") and co-adapted by William Goldman (2001's "Hearts in Atlantis"), as complicated and convoluted as "Dreamcatcher" may sound, be aware that there is more--much, much more--where that came from. A B-movie in A-list clothing if there ever was one, the film is silly, increasingly preposterous, and, finally, frustrating. Nevertheless, the visual effects, courtesy of Industrial Light & Magic, are top-notch and usually believable; the cinematography by John Seale (2000's "The Perfect Storm") is chill-inducingly beautiful and atmospheric; and the performances by a well-known ensemble are convincing and refreshingly based in reality.

At a near-epic length of 136 minutes, the opening hour is absolutely fabulous, too good to ultimately be true. Director Kasdan takes his time in convincingly setting up the four guys and their close friendship, sort of a cross between Kasdan's own 1983 classic "The Big Chill" and King's "Stand By Me." Then, as the oddball and progressively dire circumstances mount, the movie veers into a truly scary and taut rollercoaster ride, one in which nobody is safe from the chopping block. A scene involving Beaver, an aptly-named "shit weasel" trapped in a toilet, and a bunch of toothpicks especially knows how to ratchet up the tension. It may just be the most memorable and goosebump-inducing bathroom scene since Stanley Kubrick's brilliantly mounted 1980 adaptation of King's "The Shining."

Had the picture continued on this more intimate and horrific path, "Dreamcatcher" might have turned out to be one of the best Stephen King adaptations ever made. The introductions of Col. Curtis, his right-hand man, Captain Owen Underhill (Tom Sizemore), and a corkscrew away from horror and into science-fiction territory, signal that it is not to be so. As professionally shot and entertaining as it is, one cannot help but be severely disappointed by the movie's multiple-personality disorder. The sharply drawn characters are thrown away and replaced by an avalanche of special effects and generic chase sequences. The complex and slickly methodical set-up turns out to be nothing more than a ruse, as the proceedings add up to very little. And the climactic battle introduces a maddening plot development absent from the novel that, when thought about for more than ten seconds, makes almost no sense. Nevermind the absence of a satisfying last scene that should have wrapped up the characters the movie had taken some time to develop.

As misguided as the last 75 minutes are, at least someone of the stately stature of Morgan Freeman (2002's "The Sum of All Fears") is on-hand to turn in such a dignified performance as the sympathetically unhinged Col. Curtis. Freeman obviously has given the role more humanity than was found on the written page, and it makes for a surprisingly interesting character. As the four friends, Thomas Jane (2002's "The Sweetest Thing"), Timothy Olyphant (2003's "The Safety of Objects"), Jason Lee (2003's "A Guy Thing"), and Damian Lewis (HBO's "Band of Brothers") give performances more layered than they have any right to be, almost hurting rather than helping the film in the long run. Had these excellent young actors not been so good, then perhaps it wouldn't have seemed like such a betrayal when the second half thoroughly wasted them.

"Dreamcatcher" may be cluttered, occasionally confusing, and sometimes even downright bad, but it also offers up ideas and individual moments of sheer brilliance. The fantastical idea of Jonesy's Memory Warehouse is auspiciously brought to life on the screen, and is its most creative aspect. Through the good and the regretful, however, is an endless fascination that the finished product holds over the viewer. Even when Kasdan and screenwriter Goldman lets the audience down, and even when it jumbles together the spare parts of other King stories ("It," "The Stand," "The Tommyknockers"), we can't help but be entertained. Oh, how it could have been so much more than that, though.