It’s disheartening to find Scott B. Smith, who wrote the fine script for “A Simple Plan” (1998) from his own novel, falling so precipitously with this carnivorous-plant flick, whose screenplay he also adapted from a book of his. “The Ruins” is a professional enough job, with atmospheric cinematography by Darius Khondjl (“City of Lost Children,” “Delicatessen,” and “Se7en,” among many others). But as directed, ponderously, by first-timer Carter Smith, it’s at once silly, gross and boring. Though it apparently aims to be something more than a generic vacationing twentysomethings-in-peril movie, if anything its pretensions make it even more ludicrous than more lowbrow variations on the theme.

The picture begins with med student Jeff (Jonathan Tucker), along with his girlfriend Amy (Jena Malone) and their friends Eric (Shawn Ashmore) and Stacy (Laura Ramsey) frolicking on the beach in Mexico. When a German guy named Mathias (Joe Anderson) offers them the opportunity to go with him to the unchartered ruins of a Mayan pyramid, Jeff jumps at the chance, and with a Greek named Dimitri (Dimitri Boreas) tagging along, they make their way to the isolated site.

Unfortunately, they’re soon trapped atop the pyramid by a group of unfriendly locals who won’t let them leave. The temple, you see, turns out to be covered with carnivorous vines that have already gobbled up Mathias’ brother (part of an archeological expedition to the place) and, it would appear, anyone else who’d ventured there, and now target our fearful protagonists. The rest of the picture follows their increasingly desperate efforts not to be gobbled up by the unfriendly vegetation.

Many of their actions and reactions involve bloodletting, especially after one of their number is seriously injured and becomes the subject of a crude operation, and another becomes infected by the vines and goes under the knife to remove the wriggling stuff. The result is that though the picture isn’t exactly torture porn, it features a good deal of the bloody, disgusting business audiences are familiar with from the “Saw” and “Hostel” franchises. It’s just that in this case it’s self-inflicted. That doesn’t really make it any more appetizing; maybe, in fact, it makes it worse.

Malevolent plants and flowers are nothing new to sci-fi tales, of course; one need think only of “Invasion of the Body Snatchers,” “The Little Shop of Horrors” and—perhaps most memorably—“The Day of the Triffids.” Even the thing of “The Thing” was supposed to be the equivalent of a walking carrot. And “The Outer Limits” once did an episode (“Specimen: Unknown”) about malignant flowers from space, too. But all of those offered some explanation for the phenomenon. “The Ruins” doesn’t: it just posits the presence of these flesh-chomping vines in a single locale and moves on from there.

That’s okay, of course—the explanations are usually so nonsensical that none at all might be preferable. But these vines are pretty dull shrubbery. Sure, they have red flowers that, it turns out, can shriek out imitations of cell phone ring-tones (a skill that must have been learned recently and so has to be considered proof of evolution), but they creep along awfully slowly, and even their habit of insinuating themselves into bodily orifices and then moving about under victims’ skin results in visuals that are more icky than scary. (And no, none of the affected parties offers a chorus of “I’ve Got You Under My Skin,” which might have improved matters.)

But the real problem isn’t with the plants, it’s with the people. The vacationers are a typical group of giggly, oversexed beautiful things who don’t seem to manage a single thought within their collective heads. They’re such a tedious bunch that it’s not even fun to guess which of them is next on the menu. (I except poor Dimitri, who’s so obviously the equivalent of the otherwise unknown Enterprise crew member who beams down to the surface with the regular cast only to serve as a convenient victim that he doesn’t count. And none of the young performers have the chops to make any of these cardboard characters remotely interesting, though it must be admitted that Anderson provides a curious accent.

The locals are more anonymous, of course—just a gang of mean-spirited rustics determined not to allow the newcomers to escape, apparently lest they spread the plants (though it’s not entirely clear why they should think that might happen). They’re limited to glowering, shouting at the heroes in some unintelligible tongue, and occasionally shooting victims with guns or arrows. It’s a crude approximation of what anthropologists would call the confrontation with “the other,” though such a description gives the script too much credit.

And then there’s the big conclusion. “The Ruins” doesn’t so much end as just stop, though with the option of a sequel to continue the story. But it’s unlikely that enough viewers will take this unhappy journey into the Mexican wilds to justify a second expedition.