Watching Octavia Spencer go bonkers—and the Oscar-Winner does so very well—is pretty much the sole reason for expending your time and money on this goofy horror movie. Despite a surprisingly imposing cast, which includes Jennifer Lewis and Allison Janney as well as Spencer, “Ma” remains the sort of basically schlocky B-movie potboiler Blumhouse specializes in.

The catalyst for what happens in a small Ohio burg is the return of divorcee Erica Thompson (Lewis) and her high-school daughter Maggie (Diana Silvers) to what had been Erica’s hometown. Mom quickly takes a job at a casino, which will shortly take her away on extended trips to learn the art of card dealing.

That leaves Maggie to have fun with her new friends Andy (Corey Fogelmanis), Haley (McKaley Miller), Chaz (Gianni Paolo) and Darrell (Dante Brown). When trying to get an adult to buy booze for them, they find Sue Anne Ellington (Spencer), an aide to the town’s grumpy veterinarian (Janney), who, remembering her own high school years, says yes after she learns that Andy is the son of Ben Hawkins (Luke Evans), an old classmate who had humiliated her (in the flashbacks to those events, she’s played by Kyanna Simone Simpson, and Ben by Matthew Welch). Erica (in the person of Skyler Joy) she recalls as well, and the memory has apparently been eating away at her lately for reasons that will become clear as the movie progresses.

After buying the alcohol for the kids, Sue Anne turns them in to the cops for drinking it out at what appears to be an abandoned quarry, then turns around again and invites them to use the basement of her remote house for their parties. Before long the place has been transformed into a gathering spot for scads of local teens, and Sue Anne, whom they’ve come to call “Ma,” has joined in the fun, dancing and drinking along with them.

But there are signs that Sue Anne isn’t quite the happy hostess she appears to be. Occasionally she reacts angrily to the kids, and to Maggie she seems to have eyes for Andy, whom she watches with a scary stare. Pieces of the girls’ jewelry seem to disappear when they’re blotto. And there’s Sue Anne’s rule about their never going upstairs, from which occasional thuds can be heard. She blames them on a freezer kicking on, but that seems a lame excuse.

What Sue Anne is bent on, of course, is revenge, particularly against Andy and Maggie, but their parents as well. And since her boss doesn’t appear to be too careful about keeping track of her stock of animal tranquilizers and Sue Anne’s kitchen offers a good supply of knives, she has all the tools she needs ready to hand.

What gives “Ma” whatever punch it possesses is Spencer’s performance. When trying to be chummy she oozes sweetness and light, but can turn grimly threatening on a dime when anything upsets her. Spencer uses her formidable talent well through the first two-thirds of the picture, but the last half-hour predictably turns into something much nastier and explicit, and though the actress shoulders her way through the finale with the determination you’d expect, the material is beneath her.

The rest of the cast is stuck playing characters who are really quite dense, and while you can forgive the youngsters (the kids are supposed to be a callow group, after all), the adults are another matter. Still, Lewis and Evans emerge with their dignity intact. The same can’t really be said for Janney, who storms through her one-note role without the slightest nuance.

Technically “Ma” is okay for a low-budget affair. Marc Fisichetta’s production design is fine, as is Christina Varos’ camerawork. The editing by Lucy Donaldson and Jin Lee tries to cover up some of the story’s larger plot holes—there are lots of clumsy coincidences, and the revelation about Sue Anne’s upstairs secret isn’t properly prepared for—but doesn’t quite succeed; Gregory Tripi’s score is completely anonymous.

For director Tate Taylor, this represents a considerable comedown from his biggest hit, “The Help.” Perhaps he saw it as a way of restoring the luster that film gave him after the relative box office disappointment of his intervening pictures, “Get On Up and “The Girl on the Train.” Whatever else you say about it, it’s nice to see him getting support from Spencer and Janney, both of whom appeared in “The Help” (and also, along with Missi Pyle, in his little-seen debut “Pretty Ugly People”). Such shows of professional camaraderie aren’t all that frequent in today’s Hollywood.