Who would’ve thought that Tate Taylor, director of “The Help,” “The Girl on the Train,” and the series premiere of “Grace and Frankie” had a solidly gross horror movie in him? “Ma” isn’t at the level of torture porn or a slasher pic, but it is pretty gnarly at times, particularly in the violence it actually shows. This thriller is silly, skin-crawling fun that earns its R rating in a variety of surprising ways, though it isn’t nearly as explicit when it comes to clarity in its script.

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Maggie (Diana Silvers) is the new kid in school at Franklin High, after her mother, Erica (Juliette Lewis), has to return to her rural Ohio hometown after a divorce. Maggie quickly makes friends with a group of other students, led by queen bee Haley (McKaley Miller) and develops a crush on Andy (Corey Fogelmanis), whose father (Luke Evans) went to high school with her mother. The teens try to find adults to buy them alcohol, and they find sympathy in Sue Ann (Octavia Spencer), a lonely veterinary tech who went to school with their parents decades ago. Sue Ann, who likes to be called “Ma,” doesn’t just purchase the liquor; she offers the friends a place to party in the basement of her middle-of-nowhere home. Soon, the parent- and police-free zone becomes the favorite hangout of the teens to drink. However, Sue Ann oozes loneliness, and Maggie begins to suspect there’s something sinister going on with their new, older friend. You think?

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There’s little characterization in Scotty Landes’ screenplay; we don’t actually get a sense of who any of the supporting characters are and why they do what they do. These teens make profoundly stupid decisions, even for teenagers. Even for teenagers in a horror movie. No one behaves as humans do. “Ma” isn’t big on logic or details, offering little to the audience and making them do the work, like a lazy member of your Senior English group project. (I’m still mad.) In addition to the characters, plot threads feel underdeveloped, as though there was a lot left on the floor of this 100-minute movie.

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To date, Landes has largely focused on comedy, writing for shows like “Workaholics” and Sacha Baron Cohen‘s “Who Is America?” and there’s an uneasy humor here, but it’s not always clear if the audience’s laughs were the goal of the script, particularly in some of the movie’s more disturbing scenes. Even in its violent moments, “Ma” leans into comedic territory, but the tone is sloppy, lurching from gasps into giggles with little of the grace that typifies better horror comedies. We’re never sure how we’re supposed to feel at a given moment, and the direction from Taylor is little help. There are some stylish flourishes, including a few split diopter shots, and Taylor is clearly having a good time trying something new for him here, even if the results don’t always work.

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Spencer is also having a blast with the deranged character she plays. Ma/Sue Ann requires the Oscar-winning actress to turn on a dime, and she obliges. Her performance boosts the movie into watchable territory, and it’s hard to imagine it working at all with another actress here. We – and the teens – buy her sweetness and willingness to help out, at least at first, but that soon curdles into desperation and menace. It’s a manic turn that’s far from the role in “The Help” that won her an Academy Award, as well as from her work in “The Shape of Water,” “Hidden Figures,” and “The Shack,” but she proves her range here, even if the material isn’t matching her efforts.

She’s surrounded by teens who are fine in their roles, if unremarkable. Silvers gets the most to do and does it pretty well, but I won’t even remember that she was in this film when I see her in “Booksmart” for the third time. Meanwhile, Evans re-teams with his “The Girl on the Train” director and is well cast as the former high school hunk (who can still get it, and does). Allison Janney is here too, playing Sue Ann’s horrible veterinarian boss, and it’s a weirdly small role for her first big-screen outing post-Oscar, other than her previous work with both Taylor, but I’m never mad about seeing her.

Janney’s character gives Sue Ann hell, and it’s clear that it’s not the first time she’s been bullied. “Ma” is indebted to horror films of the ’70s and ’80s, most notably “Carrie,” with its warnings about the revenge of outcasts. There are some stylistic nods (those split diopter shots, of course), but it leans hardest into the vintage vibe with its musical cues. Gregory Tripi‘s score has some nice synth moments, but a lot of it comes from the diegetic music. Ma is stuck in the past, and so the party playlist she provides includes “Funkytown” and “The Safety Dance” (though there’s no protest from the kids over the old-school music, which … sure).

Despite its R rating, teens should be the key demo for “Ma.” They’re most in need of its blunt anti-bullying message, which is far more likely to convert them with its horrifying consequences than any “Be Best” campaign. Plus, Taylor’s film only really works if you turn off the rational part of your brain, which isn’t fully developed until you’re in your 20s anyway. If you can ignore the plot holes and gaps in logic, “Ma” is a fun, dumb time at the movies. [C]