The Ruins is, with one major caveat, about as good an adaptation of Scott Smith's bestselling novel as Hollywood was ever going to make. Smith's book -- about a group of college kids who stumble onto a hill in the Mexican jungle where a flesh-eating vine dwells -- was the kind of relentless beach read that seemed tailor-made for the movies, at least until you realized you were dealing with a story about a talking plant that drinks blood.

That would be fine if you were making Little Shop of Horrors. But if you're aiming for the grueling horror picture director Carter Smith is after, then the challenge becomes practically insurmountable. No matter how good your special effects crew, no matter how strong your cast, your movie is going to live or die on the whim of some shrubbery.

It's a testament to how effective The Ruins is, then, that much of the film imparts a reasonably effective facsimile of harrowing intensity as Smith's novel does. The set-up remains identical: On their Mexican vacation, two couples (Jonathan Tucker and Jena Malone, and Shawn Ashmore and Laura Ramsey) accompany a German tourist (Joe Anderson) to a remote archaeological dig to find his missing brother.

But after they arrive at the site, a hill of Mayan ruins covered by vines, the group is barred from leaving by the denizens of a nearby village, who threaten to shoot anyone who steps off the hill.

Why that is becomes clear soon enough. The bulk of The Ruins takes place atop that hill, and although it initially seems too limited a setting for anything of interest to happen, the script (written by novelist Smith) finds a constant stream of calamities to inflict upon the characters.

It should be noted that many of these situations cause them to bleed. A lot. Director Smith is as good as orchestrating sustained sequences of suspense as he is at pulling off exemplary gross-outs (a highlight: a double-amputation by rock). The cinematography by the great Darius Khondji (Seven) tempers the film's trashy B-movie roots with an air of class. And except for a stray shot here and there -- like a glimpse of the vine's tendrils making off with a severed foot -- the great potential for unintentional guffaws is mostly avoided.

It's a shame, then, that The Ruins cops out with an ending that departs from the downbeat resolution of the novel, the result of misguided studio tampering. The finale of The Ruins makes the film feel like a cheap cop-out -- a claim no one would ever make about the novel.