The runaway (if wholly undeserved) success of “The Nun” only a few months back is unlikely to be repeated by “St. Agatha,” Darren Lynn Bousman’s lead-footed tale of a convent that holds secrets much more prosaic than the ones kept in the nunnery of the earlier picture.

The change of locale is one problem. Replacing a centuries-old spooky Romanian castle with what looks like a clapboard Georgia house from the 1950s is no improvement. But then a scheme to abuse unwed mothers and profit from their newborns is hardly the equal of the machinations of a thousand-year old demon unleashed on the world, either. And replacing a cascade of special effects with a few gory moments, cheaply rendered, is unlikely to satisfy genre fans.

The plot, credited—if that’s the right word—to no fewer than four writers, centers on a girl named Mary (Sabrina Kern), who’s neither blessed nor a virgin: she collaborates with her musician boyfriend Jimmy (Justin Miles) in small-time cons. When he goes off on a gig, she seeks help at the rustic Sisters of Divinity Convent, which takes in young ladies in her condition.

But the place proves a horrible bait-and-switch operation. The Mother Superior (Carolyn Hennesy) is a brutal martinet who cows her guests into submission by, among other things, locking them up in a metal coffin. She even forces one girl to consume her own vomit on the grounds that food shouldn’t be wasted. The other nuns aren’t much better.

Mary—whom Mother Superior renames Agatha, apparently just because she can—is, however, a rebellious sort, and the movie becomes a power struggle between them, with the other sisters and their charges as secondary players occasionally drawn into the central conflict. Mary, of course, is at a constant disadvantage, even when she manages to get the local cops involved—a scene like the one Hitchcock included in “Psycho” only to prove how boring introducing the authorities into a thriller would inevitably be; Bousman more than proves the master’s point, dragging out the sequence at tedious length.

But that’s of a piece with his overall approach, which is so sluggish that it makes the movie’s ninety-minute running-time feel like twice that. As if his lethargic staging of the “contemporary” story (including a dull-as-dishwater return by Jimmy) weren’t enough, he inserts flashbacks to explain Mary’s damaged personality—an abusive father, the death of a younger brother she’d failed to protect, submissiveness in the face of a return by one of Jimmy’s victims—that are equally stately and even duller. Perhaps the pacing—brought off with editor Brian Smith—is meant to give us the opportunity to appreciate Molly Coffee’s production design and Joseph White’s cinematography, but though both are better than one might expect of such a low-budget effort, they’re not good enough to warrant so much attention. And Mark Sayfritz’s music score, with its distant choirs, just increases the torpor.

With overripe, go-for-broke but oddly stilted performances from both Kern and Hennesy—as well as the supporting cast—“St. Agatha” never generates much suspense or chills, not least because it utterly fails to capture the ambience of a genuine convent. (A nun explains at the start that the community spends most of its time in worshipful prayer and divine labor—whatever that’s supposed to mean—but there’s no evidence they even bother to put on an act.) There’s more real horror in Maggie Betts’s “Novitiate”—also set in a convent at around the same time as this film—than there is here or in “The Nun,” though it’s horror of a purely psychological sort, absent cheap theatrics or gross-out effects. By contrast “St. Agatha” comes across as just an anemic genre exercise, sloppily written, indifferently directed and amateurishly acted. It might make you pray to be delivered from more such mediocre nunsploitation pictures.