Perhaps the last thing you’d expect a reboot of “The Exorcist” franchise to botch is — you know — the exorcism scene. But that’s what “The Exorcist: Believer” does.

It’s another David Gordon Green (“Halloween” franchise) reboot, so you can expect sentimental returns of surviving characters from the original film, a renewel of the threat and an attempt or two to up the ante from that first film, William Friedkin’s chilling 1973 masterpiece, based on a William Peter Blatty novel.

Here, Green summons back Ellen Burstyn, now 90 years old and reprising her turn as Chris MacNeil, actress turned exorcism activist in the 50 years since her daughter Regan was freed from a demonic possession in Watergate Era Washington, D.C. Seeing Burstyn again is worth a lump in the throat, until you see her limited function in the story and hear some of the lines she’s commissioned to intone.

“There are many dark forces in the world, Mr. Fiedler,” she tells concerned widowed dad Victor (Leslie Odom, Jr.). “Not all of them supernatural.”

You, um, don’t say? I guess she has her reasons for accepting the supernatural as a norm.

Green’s reboot slowly introduces two tween girls (Lidya Jewett, Olivia O’Neill) to us, their dabbling in the occult and their joint disappearance for three days, a frantic local (Georgia) search that ends with them alive, but “changed.”

Green’s big contribution to this venerable franchise is two-for-one demonic possessions, both in need of exorcisms. Eventually. After medicine has run out of options, of course.

Angela (Jewett) lost her mother in the Haitian earthquake of 2010, which explains who she is reaching out “beyond.” That’s a pretty conventional motivation/”explanation” slapped onto this story, traditionally treated as simple innocence under assault. Classmate Katherine (O’Neill) comes from a devout, church-going family and she’s the one who knows rituals and summoning spells and such.

When the girls go missing, photographer and aetheist Victor is thrown in with Katherine’s equally alarmed but religiously-inclined mother (Jennifer Nettles) and pious-but-hotheaded father (Norbert Leo Butz).

And when their “answered prayers” daughters return and show “symptoms” and start acting out, it’s the religious folks who start wondering about supernatural causes. Victor has to hear that from them, and from his Catholic nurse/neighbor (Anne Dowd).

How long before they convince him to summon The Exorcist?

In the fifty years since Max Von Sydow brought an Oscar-worthy gravitas to a priest charged with performing this ritual, the movies have given us dozens of exorcists played by the likes of Anthony Hopkins, Tom Wilkinson, Stellan Skarsgard and even a Vespa-riding Russell Crowe. Not all of them played Catholic priests, but casting adds weight to such a part, no matter the faith.

Here, Green and his co-writer seek a sort of pantheistic legitimization of Catholic dogma, stripping the story of much of its Catholic mystique. Considering the half century the Catholic Church has had, its shrinking membership and its widening separation from American mainstream thought, perhaps that’s understandable.

“Believer” introduces a conservative Protestant pastor (Raphael Sbarge), a speaking-in-tongues church friend of Victor (Danny McCarthy) and an oncologist turned “healer” and perhaps voudou mystic (Okwui Okpokwasili), as well as a “hands are tied” Catholic priest (E.J. Bonilla) and that Catholic nurse. At least a lot of groups are represented here.

The film has gone to some pains to show the search for the girls, their discovery, the police and medical procedures they endure and the symptoms they start to show. Green has taken his sweet, methodical time getting us to a payoff.

And then we’re treated to a room full of faiths and non “experts,” with none of the actors — some more accomplished than others — getting across the idea that they’re seeing and experiencing something that reason would tell them “This cannot be real.”

Perhaps they’re under-reacting because they, like we, have seen too many exorcism movies to be rattled by foul-mouthed, floating or vomiting girls any more. No, “possession” effects haven’t improved that much in fifty years.

Green manages a grabby moment or two, a child busting up church communion with a Satanic shriek — “The Body and the Blood!” There’s one legitimate moment of pathos, and another achieved by stunt-casting.

But after taking forever to open up the story (some good on location footage in Haiti) and stumbling a bit in setting the “growing sense of doom” tone, Green loses the plot and with it the power to land a big third act punch. Finishing with a swing and a miss can’t have been his intent.

Rating:  R for some violent content, disturbing images, language and sexual references.

Cast: Ellen Burstyn, Leslie Odom, Jr. Anne Dowd, Lidya Jewett, Jennifer Nettles, Norbert Leo Butz, Olivia O’Neill, E.J. Bonilla, Okwui Okpokwasili, Raphael Sbarge and Danny McCarthy

Credits: Directed by David Gordon Green, scripted by Peter Sattler and David Gordon Green, based on characters created by William Peter Blatty. A Universal release.

Running time: 2:01