“Saw” (Lions Gate) is a fantastically ugly movie. If you’re easily grossed out, offended or upset by graphic violence and cruelty, stay home. If there’s even a question about it, stay home. But if you can stand to watch what unfolds, you won’t be disappointed. The film’s low budget is readily apparent, but there are enough grisly scares here that by the time you get to the movie’s end, which houses one of the best twists to ever accompany a horror film, all the trivial flaws are easily forgiven.

Two men, Adam (Leigh Whannel) and Dr. Gordon (Cary Elwes), wake up chained to the walls in a strange room, apparent victims of Jigsaw, a serial killer who forces his victims to either kill themselves or each other. The two men find instructions and a gun in the middle of the room and the game is afoot. The story follows Detective Trapp (Danny Glover) as the strain of trying to crack the case begins to take a toll on his mind, snaking back and forth from Jigsaw’s previous victims to Adam and Dr. Gordon’s present dilemma. What follows is occasionally gory, frequently tense and satisfying, almost in spite of itself.

The acting is decidedly sub-par, especially from experienced actors like Elwes and Glover. In the hands of first-time director James Wan (who co-wrote the script with Whannel), even the veteran actors fall flat, stumbling over amateurish dialogue. Nearly everything that feels off about “Saw” can be laid at the feet of its helmsman. Wan favors jittery MTV-style editing and overly gimmick-laden, unoriginal camera shots where simplicity would have been the best course.

What does work here, consistently and to great effect, is the film’s sense of pacing and its structure. Wan and Whannell and superb storytellers who have their first work hampered by some amateurish mistakes. That this is still one of the scariest, most haunting films in years is a testament to just how good a story “Saw” tells.

“Saw” opens in Washington, D.C. Friday, Oct. 29. n