As a die-hard fan of the original “Exorcist” (1973), I’m always mystified when another remake appears on the scene. It’s like trying to redo “Citizen Kane,” the first iteration was monumental, and any attempts to live up to it are predestined for failure. But don’t take my word for it, go and see “The Last Exorcism,” the second feature from German director Daniel Stamm (“A Necessary Death”). Perhaps producer Eli Roth, erstwhile author of the “Hostel” series, figured that his target audience was young enough never to have seen the original “Exorcist” and thus wouldn’t know what they were missing. For the rest of us, though, the comparison is pretty stark.

The narrative of “The Last Exorcism” plays out as you might expect—almost. This postmodern mockumentary (shaky camera and all, folks) follows the cynical pastor Cotton Marcus (Patrick Fabian) as he performs his last exorcism, intending to expose the practice as a fraud. Cotton and the film crew journey deep into the creepy wilds of rural Louisiana and meet the Sweetzers: the heavy-drinking father (Lewis Herthum), the clearly disturbed brother (Caleb Landry Jones) and the overly angelic teenage daughter, Nell (a scene-stealing Ashley Bell). Father Marcus goes ahead with his sham exorcism and thinks that that’s the end of it—then Nell displays some truly superhuman abilities, and he is forced to rethink everything he knows about God, the Devil, and reality.

At this point, things look fairly promising—you’re imagining that Marcus will be forced to confront the reality of demons and possession, and that he will save Nell and come out of the experience with his faith restored. Instead, in the last five minutes of the film the writers apparently decided they weren’t up to the task of tying up loose ends, and stuck on a finale that feels even lamer than the psychiatrist scene at the end of “Psycho.” The inadequacy of the ending really can’t be overstated: it’s like a mash-up of “Rosemary’s Baby” and “The Blair Witch Project,” but worse. All the work that went into setting up the story is thrown away at the end and the film loses whatever potency it managed to generate in the process.

“The Last Exorcism” does have some redeeming features. Chief among them is an interesting and surprisingly coherent implication that “possessed” Nell is actually being sexually abused, and that this accounts for her unstable personality and terrifying nocturnal hijinks (she is particularly fond of slashing the throats of her father’s farm animals). After witnessing Nell’s distress, Father Marcus is quick to hand her over to science—he repeatedly tells the father that Nell needs psychiatric treatment, but this, of course, is unheard of. The only salvation she needs is the Lord, and thus Marcus is drawn deeper and deeper into the situation, until its regrettably stupid conclusion. It’s also worth noting that the actual possession scenes are surprisingly restrained, for a Roth-produced picture; there aren’t buckets of blood and absurd CGI, but there’s certainly enough terror and weirdness to keep things interesting.

If the realistic part of the story—that Nell was being abused and was never in the grip of a demon—had actually been followed through to the end, “The Last Exorcism” could have been a culturally relevant and provocative update on the original adolescent-girl-possessed story. Instead, by totally departing from everything it’s worked to set in motion, the film’s ending simply glosses over all its own implications and leaves no room for interpretation. What could have been new and daring ended up being nothing but hugely disappointing but for those of us who are loyal to the original film it’s hardly a surprise.