How do you do, fellow kids?

While the category of “horror” is broad enough to include “Ma” (especially the last 20 minutes of it), you’ll be disappointed if you’re expecting anything scary or suspenseful. It’s more of a psychological drama and a mystery with some horror elements thrown in. You might still be disappointed, though; the movie would have done well to choose a more decisive path and commit to it.

Directed by Tate Taylor from a screenplay by first-timer Scotty Landes (a TV comedy writer), “Ma” reunites Taylor with his “The Help” Oscar-winner, Octavia Spencer, who plays Sue Ann, a lonely, middle-aged veterinary assistant in a small Ohio town who befriends a group of teens and becomes their booze-supplier and party-enabler (her isolated country house has a sweet basement). The main teen and the movie’s co-protagonist is Maggie (Diana Silvers), who has just moved to town with her casino-waitress mother, Erica (Juliette Lewis), who grew up here. Maggie is sweet on Andy (Corey Fogelmanis), a nice boy whose dad (Luke Evans) was BMOC back in the day and is now dating the skanky Mercedes (Missi Pyle). All of the adults know each other because they went to high school together, and Sue Ann has flashbacks to that era that shed light on her current mental state.

She starts out merely friendly, if grossly irresponsible and a little randy. The teens — Maggie, Andy, cool girl Haley (McKaley Miller), hot guy Chaz (Gianni Paolo), and token minority Darrell (Dante Brown) — can’t believe their good fortune at finding a pliable grown-up who will buy them alcohol and let them drink and smoke in her basement. (These parties soon grow to have dozens of attendees, but it’s not clear how Sue Ann recruited them.) They don’t really think of her as a “friend,” though, which becomes a problem as Sue Ann grows clingy, needy, and manipulative, on her way to full-on unhingedness.

It takes a while for that to happen, but time passes pleasantly enough while we wait for it. Maggie and Erica have a mother-daughter comradeship that’s unusually healthy for a teen movie. Maggie and Andy are a cute, wholesome couple. Allison Janney (who was also in “The Help”) does the director a favor by showing up for a few brief, amusing scenes as the exasperated veterinarian Sue Ann works for.

When things finally do go bonkers, they go just bonkers enough to make you wish they’d gone more bonkers. This is a trashy, exploitative B-movie at heart, but it hedges its bets by keeping one foot in the realm of respectability. There are legitimate themes that could have been dealt with here — bullying, peer pressure, multi-generational dysfunction, dead-end small towns — but only if they were treated seriously and not surrounded by such a gleefully ludicrous plot. What can I say? It should have been trashier.

C+ (1 hr., 41 min.; R, a lot of harsh profanity, some nonsexual nudity, brief mild sexuality, some violent images.)