With pretzel-knotted limbs, a disturbing flow of menstrual blood and a baptism scene of unnerving quiet and unswerving menace, The Devil Inside is a refreshing R-rated reversal of PG-13 puniness in many recent possession films — The Exorcism of Emily Rose, The Last Exorcism or The Rite.

But balls only get you so far. Yes, “The Devil Inside” is rarely timid. But boy, Captain Howdy, is it almost always stupid — especially in a contemptuous conclusion certain to collect catcalls across the nation. Arriving after just 73 minutes, it’s a finale dumber than all of the found-footage folderol since The Blair Witch Project and an argument for euthanizing this subset of the horror genre altogether.

The best that can be said of it is that the finale forgoes a demonically possessed woman lunging at the camera as a boar would a bunny — if only because that effect occurs twice during the first 10 minutes.

After a silly wink-wink warning that the Vatican didn’t endorse this film, a woman leaps from a crawlspace at cops combing the scene of a triple homicide in 1989. The subject of an aborted exorcism, Maria Rossi offed everyone in the room, then called 911 to confess.

Diagnosed with dissociative identity disorder and spared prison on an insanity plea, Maria is institutionalized … but swiftly sent to a mental hospital in Rome. (And if you’re wondering how someone institutionalized in the States could be extradited to a run-down Roman sanatorium, well, you’re putting more thought into it than co-writer / director William Brent Bell and co-writer Matthew Peterman.)

Two decades later, we meet Maria’s daughter, Isabella (Fernanda Andrade, hiding a Brazilian accent in a monotone). Isabella worries Maria’s mental condition may run in the family and wants to see Maria for herself. Chronicling her trip is documentary filmmaker Michael Schaeffer (Ionut Grama, Romanian for B.J. Novak).

And it’s certainly a strange and mournful day for this mother and child reunion — Maria passing judgment on an abortion she couldn’t possibly know Isabella had. As Maria, Suzan Crowley (a nicely coincidental surname) generates effective, expressive madness with a crinkled face, croaked voice and bloodcurdling screams.

So far, so good. But then Isabella and Michael meet Ben (Simon Quarterman) and David (Evan Helmuth) — a pair of priests in exorcist school (no joke) who have hung a shingle for exorcisms the Catholic Church refuses to perform.

In a relentlessly gabby second act that grabs at straws of religious relevance, Bell and Peterman’s script becomes possessed with fatal self-seriousness. It doesn’t help that Quarterman is a spastic shouter and Helmuth, though genial, seems the sort better suited to play a Prilosec patient than a priest.

Much better is a simple, suspenseful, very bloody freak-out at a basement exorcism on a girl played by actress Bonnie Morgan and contortionist double Pixie Le Knot — another nicely coincidental surname, whose wet, crunching sounds of bone displacement may have you gritting your teeth.

Now a believer, Isabella persuades Ben and David to cast out her mother’s suspected demons. It’s no surprise when that goes south. What shocks is just how much laughable dialogue, lamentable plotting and unabashed brainlessness ensues.

Why, again, would a mental hospital permit an exorcism on the premises? Why can’t priests who own cameras to track “preternatural pupil dilation” afford stronger restraints than … cloth strips? Eventually, The Devil Inside throws its hands up and becomes Real World: Vatican City, or what happens when people stop being polite and start getting possessed.

Bell knows how to stage a handful of fine freak-outs but can’t tie them together into a seriously sinister story. Unfortunately, he doesn’t do enough to elevate the heart rate in this exorcise routine.