In its brighter moments, The Last Exorcism draws on ambiguity for its suspense. Is the teenage girl under suspicion actually demon-possessed or merely the victim of poor home schooling? For a while it could swing either way, and the tension created by this approach (previously tested, with a higher degree of success, by Scott Derrickson in The Exorcism of Emily Rose) is entertaining, even philosophical. But the strategy is conveniently dropped for a sensational ending that belittles the hard work that comes before it.

Shot in an unsteady, blurrily-ocused style which should be familiar to all viewers of contemporary television, the film purports to be a documentary made at the request of the main character, a Baton Rouge preacher (Patrick Fabian) partly inspired, the filmmakers say, by con artist Marjoe Gortner. Even though he claims to have lost his faith in the divine long ago, he continues to serve at his church, supplementing his income by administering faux exorcisms the way a doctor administers placebos. The film crew, consisting of an unseen male cameraman and a female boom operator, intend to follow him to an isolated Louisiana farm where he will expose a case of demonic possession as fraudulent. Or is it?

The early scenes with the preacher, Cotton Marcus (conceivably named after the famous New England minister and witch hunter Cotton Mather), are unexpectedly offbeat as he cozies up to the camera, boasting of his snake oil salesmanship. (Why the congregation remains oblivious to this charlatan's beliefs while his immediate family appears totally at peace with them is never explored.) The success of these early scenes is largely a matter of personal taste, as some will not find the idea of a phony preacher funny in the least, although Fabian's charisma in the role gives Cotton an uncomfortable charm.

A sense of expectancy builds as the crew approaches the backwoods farm and meets the afflicted family: the God-fearing but hard-drinking father (Louis Herthum), his casually hostile teenage son (Caleb Landry Jones), and finally, the sweetly naïve 16-year old daughter (played with maximal dedication by 24-year-old Ashley Bell) allegedly under the influence of Satan. It's encouraging that the filmmakers, particularly director Daniel Stamm, are disciplined enough to abstain from unleashing the scary stuff for the first 45-or-so minutes—more than half the film's modest runtime. One looks forward in pleasurable anticipation to the promised thrills, and the wait is rewarded by two or three well-timed jolts.

But Exorcism backslides into the routinely ridiculous soon enough. The discovery of prophetic drawings, the bloody stabbing of a household pet, an unexplained pregnancy—the plot thickens, but never plausibly. And the rigor of the documentary approach is compromised by the variety of angles (which suggests the work of several cameras, not one) and the presence of a generically spooky score.

Finally, after a few mild tantrums and false alarms, the film builds to a vivid but perplexing climax more suggestive of the Hammer horrors of the 1960s than anything consistent with Reality TV. By this point, any loss of faith experienced by the viewers will likely be directed toward the filmmakers, not God.