If you tried to describe the demented plot of Scott B. Smith's 2006 bestseller The Ruins to friends or family members, they'd probably think you were making the whole thing up. Fusing elements of Alex Garland's The Beach and Agatha Christie's And Then There Were None with a liberal dash of Little Shop of Horrors tossed in for good measure, The Ruins follows four twenty-something American tourists who decide to spend the last day of their Mexican vacation visiting some Mayan ruins, where they unexpectedly become food for—wait for it—a sentient bloodsucking plant.

Sounds patently absurd, right? Well, read the first few chapters of the book and you'll feel that grin vanishing right off your face. From the opening paragraphs, Smith, who previously topped the bestseller chart with his 1993 thriller A Simple Plan, suffuses the novel with a palpable sense of dread that offsets the ridiculous storyline. It helps, too, that the book's most horrifying moments involve things that the characters do to one another or themselves, rather than any otherworldly threat. Reduced to its essence, The Ruins is a positively Herzogian tale about survival and madness in the heart of the jungle.

It's a shame, then, that Werner Herzog didn't direct the movie version of Smith's book. Instead, the assignment fell to first-time feature filmmaker Carter Smith (no relation to the novelist) and while he proves himself technically proficient, his film is drained of any of the suspense that distinguished the novel. To be fair, part of that is due to the workmanlike screenplay—penned by the novelist himself—which follows the broad outline of the book, but makes the mistake of emphasizing the monster over its victims. Instead of the richly imagined personalities we met on the page, the film presents us with standard horror-movie archetypes, including the straitlaced leader (Jonathan Tucker), the screaming ingénue (Jena Malone), her flighty best friend (Laura Ramsey) and the laid-back party guy (Shawn Ashmore). And where the characters' behavior drove the book's narrative, the movie allows them to do little more than react to external circumstances, making it difficult for viewers to feel invested in their fate. The film's biggest sin, however, is its ending, which completely violates the spirit of Smith's novel and doesn't even make sense in the context of this version of the story. Either the author was well-compensated for signing off on this change or it was filmed in secret and he still hasn't been informed.

Even hardcore horror fans would have to concede that the genre is going through a rough patch right now, as the major studios continue to glut the marketplace with pictures that share interchangeable titles, plots and casts. What's so distressing about The Ruins is that DreamWorks had a real chance to make a movie that stood apart from the pack. Traces of that film can still be glimpsed onscreen, but they are largely drowned out by the crushing sameness that makes a genuinely unique story like The Ruins seem as derivative as the upcoming Prom Night remake.