"The Ruins" has been billed in its advertising as a genre piece akin to 2006's "Turistas" or 2006's "Hostel," with young American vacationers befalling an unimaginable nightmare amid foreign surroundings. While this is technically true, anyone who has read Scott B. Smith's disturbing bestselling novel of the same name will be well aware that the story has more than a few tricks in its arsenal, including an imaginatively vicious villain more devious and deceptive than an organ harvester or a masked maniac with a butcher knife. Where Smith's source material really stood out, however, was in its psychological study of people pushed to the thresholds of their own sanity as they fight for survival. An unflinchingly realistic human tale colliding with some wildly scary supernatural elements were precisely the right ingredients for the novel to earn its power.

As tautly directed by Carter Smith (2006's well-received short film "Bugcrush"), "The Ruins" is a tonally and subjectively honest adaptation that no doubt was aided by author Scott B. Smith, who also is the screenwriter. While he has shaved off some of the depth of his characters in order to meet a 90-minute running time, neither writer nor director Smith have lost sight of the bigger picture. More often than not, their fat-trimming is beneficial to the simplicity of the storytelling, the pace a gradual but always arresting climb toward a second half that gets more brutal, desperate and unsparing with every new scene. And then, in a veritable flash, the movie almost destroys itself with a tacked-on and cowardly ending that diverges for no good reason from the novel's vastly superior and bleakly logic-driven conclusion. This misbegotten denouement turns what could have stood as a headier, more profound motion picture into just another run-of-the-mill horror tale.

Best friends Amy (Jena Malone) and Stacy (Laura Ramsey), along with boyfriends Jeff (Jonathan Tucker) and Eric (Shawn Ashmore), are recent college graduates letting loose on an exotic Mexican vacation. When fellow foreign traveler Mathias (Joe Anderson) explains that he is headed to an archaeological site in the jungle where his brother has gone missing, the four of them offer (with different degrees of willingness) to accompany him on his journey. Once they step foot in the greenery surrounding a Mayan temple, they are cornered by weapons-carrying tribesmen who refuse to let them leave alive. Now stranded atop the ancient ruins with no workable cell phone and a dwindling supply of food or water, Amy, Stacy, Jeff and Eric are about to discover that their straits are infinitely more dire than they initially expected.

In essence, "The Ruins" follows several fateful days in the lives of four average young adults—days that not only squash their hopes, dreams and aspirations for the future, but leave them questioning whether they have any future at all. The characters, though not as three-dimensionally realized as they could have been, still come off as a whole lot more authentic and naturalistic than the horror genre usually allows. Partially because of this, and partially because director Carter Smith depicts each situation in a matter-of-fact style that doesn't try so hard, the viewer is quickly drawn into the characters' extreme plights and buys into the more fantastical aspects of the plot.

Set-pieces of heightened, almost breathtaking intensity arrive at a sure-headed clip, no more so than in a sequence where Amy and Stacy are lowered into a deep, cavernous shaft to locate a cell phone they hear ringing. Expertly shot and edited, the film provides genuine thrills and frights without lowering itself to cheap jump scares and an overabundance of hokey CGI effects. The graphic violence, some of it self-inflicted, is beyond grisly, but isn't exploitative; it's there for a reason, and adds to the pitch-black tone. Kudos, too, for one of the eeriest sound designs in recent memory; it works its way underneath the audience's skin in just the way intended.

The performances, especially those turned in by Jena Malone (2007's "Into the Wild") and Laura Ramsey (2006's "The Covenant"), rely on heightened emotions with necessary moments of levity and empathy. Malone makes for a lead that stands out from the crowd; she turns Amy into a fun-loving but decidedly opinionated type who also happens to be quite intelligent. She isn't a vision of virtue, or perfection, but a normal girl whose personal defining traits allow for a more indelible, less obvious heroine. Ramsey, meanwhile, dives full-throttle into the demands called upon from her tragic character of Stacy, whose mental collapse coincides with some, shall we say, unfortunate physical maladies. Her acting in the latter scenes are a tour de force of intensity. The male actors aren't as well-defined, with Jonathan Tucker (2007's "In the Valley of Elah") and Shawn Ashmore (2006's "X-Men: The Last Stand") falling to the wayside as the ladies get their hands dirty and do all the heavy lifting.

A horrific, edge-of-your-seat experience in the same way that 2006's "The Descent" was, "The Ruins" is additionally effective for not overexplaining the unstoppable adversary the characters face. We know as much as the people onscreen know—that is to say, not much—and sometimes the scariest things are those that remain enigmas. Because the film works so splendidly for so long, it is with a regrettable heavy heart that the final scenes come close to ruining the whole enterprise. Since the screenplay was adapted by the author, it is truly bizarre that Scott B. Smith would so drastically alter the outcome of the story and, thus, lessen the haunting thematic notions of his novel. Even taken on its own terms, this wrap-up is superficial and frustrating. The sole misstep that "The Ruins" takes is unfortunately a major one, ending on a note of deflation and disappointment. What comes before this is superbly crafted and as tight as a vise grip, worth seeing for that alone. What a shame those last few minutes are, though.