Scott Smith's novel "The Ruins" is a great thriller, with the chills evenly divided between those of a psychological nature and those that are more graphic. The film version, adapted by Smith himself and directed by Carter Smith (no relation), removes most of the psychological aspect and goes right for the visceral, gut-churning thrills. And you know what? It may not be as fundamentally unsettling as the book was, but it's not a bad horror flick.

The story, told briskly and without many extraneous details, concerns four American college students on vacation in Mexico. Jeff (Jonathan Tucker), the responsible one, is heading to medical school soon; his girlfriend, Amy (Jena Malone), drinks too much and flirts a lot. Amy's best friend is Stacy (Laura Ramsey), who is a perfect match for her own boyfriend, Eric (Shawn Ashmore), because neither one has any distinct personality traits of any kind.

The four meet a German tourist named Mathias (Joe Anderson), and he tells them about an archeological dig out in the jungle that his brother has gone to check out. His brother left a map. Would the four Americans like to join him as he goes to see some cool ancient Mayan artifacts?

All five of them head for the jungle, joined by a Greek fellow whose name we don't learn until after it's too late to be useful. (If this were "Star Trek," the Greek would be wearing a red shirt instead of no shirt at all.) They find an old Mayan temple, and sure enough, there are indications that Mathias' brother and the archeologists have been here. But where are they now? And why has a group of well-armed local Mayans encircled the temple and forced our heroes to climb to the top of it and stay there?

I won't say any more about what happens except to give some generalities. Things go awry almost immediately. There is a threat to Mathias and the Americans beyond just the armed Mayans. The film toys briefly with exploring the psychological threat that everyone poses to everyone else -- suspicion, panic, human nature, etc. -- but as I mentioned earlier, most of that has been excised. It would have made the film better, but it also would have made it longer, and perhaps the wiser choice was to keep things moving, especially if there was to be so little characterization. But now my reasoning has become circular.

I'm not gonna lie to you: This movie is gross. Some characters endure painful injuries, followed by potentially life-saving medical procedures that are arguably worse. The film doesn't shy away from the gory details, though I wouldn't say it revels in them, either. There's a grim matter-of-factness about everything: Here are some people, here is what happened to them, the end.

You may recall that Scott Smith also wrote the book and screenplay of "A Simple Plan," another story about people getting into a situation they cannot control that only gets worse the more they try to fix it. "The Ruins" is definitely a step down from "A Simple Plan," but it is reasonably well executed. I certainly won't be exploring any Mayan temples anytime soon, that's for sure.

Grade: B-

Rated R, some harsh profanity, a lot of very strong violence and gore, brief nonsexual nudity

1 hr., 31 min.