C-

“The Exorcist” meets “The Blair Witch Project” and gives birth to “Rosemary’s Baby.” That’s the strange brew producer Eli Roth and director Daniel Stamm have concocted in “The Last Exorcism,” a cheesy horror movie in the now commonplace verite style that manages a few easy jolts but overall isn’t very scary, and suffers from a terrible last-act twist. It obviously wants to be another “Paranormal Activity” type of phenomenon, but won’t succeed.

The first section of the picture takes mockumentary form to introduce Cotton Marcus (Patrick Fabian), a young evangelical preacher who cheerily admits that he’s lost his faith and intends to get out of the business. But before he does, he’ll allow the filmmakers—headed by Iris Reisen (Iris Bahr)—to record one last exorcism, which he picks out from the mail he’s just received. Before long he’s traveling into rural Louisiana to perform the ritual—complete with parlor tricks—on Nell Sweetzer (Ashley Bell), whose father, a credulous farmer named Louis (Louis Herthum), is convinced she’s possessed and killing their livestock. The preacher persists in driving to their isolated spread despite being warned away by the girl’s hostile brother Caleb (Caleb Landry Jones).

Cotton goes ahead with the ersatz exorcism and declares the girl cured, but a problem soon arises. Nell is still extremely disturbed, and actual diabolic activity appears to be continuing. When it’s discovered she’s pregnant, questions about the paternity necessarily follow, and suspicions of incest and child abuse as well. Violence also occurs, along with threatening prophecies. And though Iris and her cameraman doubt they should stay, Cotton refuses to leave, insisting they see the case through to the end. And he enlists the help of the local minister (Tony Bentley), despite the fact that Louis had stopped going to his church some time earlier. It turns out to be an unwise decision.

The plot concocted by Huck Botko and Andrew Gurland doesn’t make a whole lot of sense, particularly after the big concluding twist that most viewers will find as ridiculous as the similar one in the 1988 misfire “Spellbinder.” But they and Stamm succeed in concealing the holes somewhat through the verite style, with its jerky, hand-held camerawork. The technique is frankly hackneyed and overused by now, but it’s probably the only way the material could be kept from becoming utterly laughable. It also papers over the mediocre quality of the acting, which only Bell exceeds with a few genuinely creepy moments. One does wonder about the accents, though. The setting is supposed to be the most rustic part of Louisiana, but except for a few incidental characters the crew talks to along the way, everybody speaks with flat, unaffected tones that sound more like drab Midwestern voices.

“The Last Exorcism” will also offend some with its anti-religious emphasis. Not only is Marcus an admitted charlatan more interested in fleecing his flock than tending to them, but Louis’ fundamentalist beliefs come in for scorn, and when it comes to Bentley’s ostensibly mainstream church, the less said the better (though his chubby female aide—or is she supposed to be his wife?—is an amusingly cherubic character). There’s nothing wrong with such a dismissive attitude, of course, but in the end it all doesn’t amount to much.

And neither does the movie.