For about 75 minutes, "Legion" is a solid, pulpy B-movie along the lines of 1986's "Maximum Overdrive" and 1990's "Tremors," a horror actioner wherein a group of disparate people are forced to hole up in a desolate setting and protect themselves from an outside threat closing in. Scott Stewart, a visual effects artist making his directorial debut, capably sets up his players at the onset and then tosses a seemingly indomitable conflict upon them that could very well spell the apocalypse. The screenplay by Stewart and editor-turned-cowriter Peter Schink is full of clichéd dialogue, but it tonally works with the piece. The actors do well, bringing more to their characters than the genre usually allows. The pace, despite alternating between scenes of soul-searching chit-chat and ghoulish, guns-blazing set-pieces, keeps relatively involving as the viewer patiently waits for the plot gaps to be filled in and the narrative to lead somewhere of note. Alas, they don't, and it doesn't. The film, signaling unmistakable post-production tinkering and amateurish editing, crashes and burns during an anticlimactic third act that answers none of the story's pertinent questions, solves none of its mysteries, and makes no sense.

On the edge of the Mojave desert, in a place called Paradise Falls—a sort of cafe, filling station and auto shop in one—a ragtag clan of employees and passersby are besieged by a lurking evil from outside. Soon after the phones, televisions and radios stop working and a demonic, wall-crawling elderly lady (Jeanette Miller) attacks, they are paid a visit from Michael (Paul Bettany), an angel who has decided to go against God's wishes and save humanity from the end-of-days invasion He has begun. The key to the earth's survival lies with the unborn child of waitress Charlie (Adrianne Palicki), whom all the other angels want dead. If they succeed, it will cement the fall of mankind. Why is this? Who knows? Also figuring into the fight are father and son Bob (Dennis Quaid) and Jeep Hanson (Lucas Black), the latter with a love and desire to protect Charlie; cook Percy Walker (Charles S. Dutton); lost traveler Kyle Williams (Tyrese Gibson), and a family with car trouble, parents Howard (Jon Tenney) and Sandra Anderson (Kate Walsh) and teen daughter Audrey (Willa Holland).

What has happened to the outside world at large? Why is Charlie's baby so important? How come God can't make up His mind if He wants to destroy the world and murder the human race or save it? And, say—why is Sandra inexplicably tied to a chair during the second half? One doesn't see her get tied up, mind you; she just suddenly is like that. Another major character is abruptly knocked off off-screen, her fate treated as a passing dialogue afterthought. "Legion" is an example of a motion picture that has obviously seen extensive reshoots and gone through multiple iterations via extensive cutting and re-editing. Certainly director Scott Stewart couldn't have originally intended for his film to end as it does, in a disastrous hodgepodge of vague, unfocused plot developments and unanswered questions. Indeed, the whole point of the movie fails to be explained. The viewer never learns why Charlie's baby is so important, why she is chosen out of all pregnant women, and why it is so vital that the child survive when, after all is said and done, presumably plenty of people are still alive across the land. If the intention was to suggest that the entire population has been wiped out, then that should have been established. Not surprisingly, it isn't.

Before incoherence and implausibility set in, "Legion" shows a lot of promise. The characters, archetypes all of them, are nonetheless likable and given enough back-story to individualize themselves. The setup, building dread little by little, is absorbing and well-constructed. The sequence involving the kindly old lady who transforms into an obscenity-spewing monster is auspicious and creepy. Another scene centering around a possessed, shape-shifting ice cream man (Doug Jones), the playful, childlike tune over the truck's loudspeaker announcing his ominous presence, is also pleasingly twisted. For once, the survivors aren't decided upon based on who gets top billing. Of the talented performers, Adrianne Palicki (TV's brilliant "Friday Night Lights") is the highlight as heroine Charlie, authentic and dazzling in her portrayal of a confused young woman who finds the will to take responsibility and fight for a baby she wasn't sure she even wanted to keep originally. Headliner Paul Bettany, by comparison, never quite grasps how to play angel Michael. He comes off as stiff and disconnected from the rest of the film, and turns in a significantly more accomplished turn in 2010's "Creation."

It is sorely disappointing that "Legion" gets so messy and goes so wrong in the home stretch. It had what it takes to stand as an enjoyable thriller, but instead has become a frustrating experience with half-formed ideas, sloppy bookending narration, and editing that must have been done with jagged scissors. Whoever is responsible for the final cut must get the brunt of the blame; their ruinous handprints taint a film that is perfectly efficient most of the time and disastrously inept for the remainder. Sadly, a far better, more cohesive picture probably exists somewhere on the cutting room floor.