The “Conjuring” franchise continues its march backward in time with “The Nun,” which serves as a prequel to the two “Annabelle” movies that were prequels to the original pair of “Conjuring” pictures. How far back this process will eventually take us is unclear—maybe to the emergence of life on earth—but this installment is rather like what the “Exorcist” movies would have become if they had gone back to the origin of Pazuzu.

Though Vera Farmiga, Patrick Wilson and Lili Taylor appear briefly at the beginning and end of “The Nun” to connect what’s happening to the “Conjuring” movies, the real link here is the not-so-sisterly spectral figure adopted by the demon Valak in “The Conjuring 2” and “Annabelle: Creation.” It (or she, if you prefer) is the force haunting the remote Romanian abbey of St. Carta in 1952. A prologue shows how its malevolent power was instrumental in the suicide by hanging of Sister Victoria (Charlotte Hope), one of the members of the cloistered community, whose bloodied body was discovered by Maurice Theriaut, aka “Frenchie,” a local tradesman—actually a transplanted Canadian (Jonas Bloquet)—dangling from an upper widow of the medieval building.

The script by Gary Dauberman will make a half-hearted effort to explain how Valak was first brought to the castle that is now St. Carta, how it was sealed away for centuries, and how it was recently released back into the world. But its main focus is on a modern investigation into the circumstances of Sister Victoria’s recent suicide by an unlikely team sent by the Vatican—one is an experienced miracle hunter Fr. Burke (Demian Bichir), but the other, Sister Irene (Taissa Farmiga, Vera’s sibling) is merely a novice who has not yet taken final vows in her order. (We will later learn why the Vatican chose her for the job on the basis of her childhood experiences. Apparently there are no secrets within the inner workings of the papal curia—depicted in a cheekily humorous early scene featuring David Horovitch and Michael Smiley as manipulative prelates there.)

Though some of the younger nuns—most notably Sister Oana (Ingrid Bisu)—are relatively forthcoming about the goings-on at the abbey, the veiled, crone-like Mother Superior (Lynnette Gaza) is not, and so our two intrepid seekers after truth are compelled to sneak around to ferret it out on their own. Of course they will each have to endure some scary episodes—spooky apparitions, weird dreams, recollections of past failures, even in one instance a premature burial—on the way to a grand finale, a prolonged confrontation with Valak that involves a mysterious key and the holiest of all relics.

The best thing about “The Nun” is the successfully gloomy, forbidding ambience fashioned by production designer Jennifer Spence that makes good use of religious iconography, even if cinematographer Maxime Alexandre films everything in such profoundly dark tones that visual murkiness inevitably results. And certainly Bonnie Aarons’ apparitions as the sinister title figure are as effective as they were in smaller doses in the earlier pictures in the series. They’re like eerie panels from a graphic novel.

But that has to be weighed against the choppy nature of Dauberman’s plot and the pro forma execution by Alexandre, helmer Corin Hardy and editors Michel Aller and Ken Blackwell. They’re aiming for a roller-coaster ride, but it turns out to be one where the curves quickly come to feel repetitive and increasingly ineffectual. There are only so many times when a ghoul suddenly appearing behind somebody and touching their shoulder can be startling, or even mildly unsettling. The sudden disappearance of background figures can’t work when repeated so often. Jump shocks grow progressively less frightening when they occur over and over, even when accompanied by loud bursts from Abel Korzeniowski’s brooding score. There are ways too many sequences of characters meandering about with lanterns, waiting for the next burst of action to occur. Even that “buried alive” sequence seems to go on forever—and makes little sense anyway. The final face-off with Valak is similarly overextended, involving way too many shots of people being slammed against walls and strangled, and the CGI effects in it are hardly top-drawer.

Still, one has to respect Bichir and Farmiga for getting through it all with straight faces; he’s pretty boring, to tell the truth, but she has a pleasantly girlish quality that makes Sister Irene a sympathetic sort. It’s Bloquet, however, who will probably be the audience favorite; Frenchie has a womanizing side that he happily expresses even toward Irene, and delivers most of the movie’s much-needed doses of humor. Bloquet embodies the character nicely, so much so that you might be nonplussed by his last scene.

“The Nun” will undoubtedly draw in audiences hooked on the franchise, but on its own it’s a pretty standard-issue example of religiously-based gothic horror. If asked how many genuine scares it offers, it wouldn’t be fair to do a pun on the title and reply “none,” but a response of “too few” would certainly be accurate.