It's about time Octavia Spencer received top billing in a major motion picture. It's been nearly a decade since the veteran character actress broke through with an Oscar-winning turn in 2011's The Help, as a free-spirited maid who overcomes the degradations of the Jim Crow South. Two additional Oscar nods and many supporting roles later, her protean lead performance is the best thing about Ma, a horror and teen comedy mash-up that falls prey to tired genre tropes.

The film opens with 16-year-old Maggie (Diana Silvers) and her mom (Juliette Lewis) en route to the elder's Ohio hometown following a failed marriage in San Diego. The daunting prospect of a new school awaits Maggie, but within minutes on her first day she connects with a group of attractive cool kids whose sole objective in life appears to be securing booze for drinking sessions at a local rock quarry.

Enter Spencer as a lonely veterinary assistant named Sue Ann, who wanders past a liquor store and agrees to buy a few bottles for the thirsty teens. Things soon get weird. Sue Ann begins stalking the teens on social media. She invites them to her house, where they call her "Ma" and get sloshed in her basement. Ma has just one cardinal house rule: Don't go upstairs. Of course, Maggie and a friend inevitably ascend the stairs, where they discover a dark secret about Ma in a suspenseful scene inspired by Alfred Hitchcock's Psycho.

The premise of a group of high schoolers repeatedly hanging out with a sketchy middle-aged woman is more than a little far-fetched, but Spencer's charismatic screen presence makes it seem at least somewhat plausible. Her performance shifts seamlessly from endearing humor to creepy menace, often in the same scene.

Silvers, who had a key supporting role in the recent coming-of-age comedy Booksmart, is the other standout in the cast. She has expressive eyes and a natural screen presence that bodes well for her future in Hollywood.

Ma is Spencer's fifth collaboration to date with The Help director Tate Taylor, who most recently cast her as brothel madam Aunt Honey in the 2014 James Brown biopic Get on Up. Taylor and screenwriter Scotty Landes are clearly aiming for an edgy John Hughes vibe in the film's many party scenes (the Hughes-penned Pretty in Pink is even referenced in the dialogue), but with the exception of Maggie, the teens are too one-dimensional for us to care about.

Throughout his career, Taylor has shown a fondness for nonlinear storytelling. The problem is, he isn't particularly adept at weaving flashbacks into a present-tense narrative. In Ma, the backward leaps in time gradually reveal a Carrie-like prank from Sue Ann's high school days that provides an explanation for her increasingly erratic behavior. But was the incident racially motivated? Was it merely because she was shy and wore thick glasses? The filmmakers never adequately explain, leaving the flashbacks dangling as empty plot devices.

Two of the best horror films in recent memory — Jordan Peele's Get Out and Us — used genre conventions to probe salient contemporary issues of racism and classism. In Taylor's film, horror clichés merely get in the way of what could have been an interesting examination of adolescent bullying and the lingering psychological scars that persist in adulthood.

Instead, as the movie's comedic elements dissipate and the story lurches toward its gory climax, even Spencer can't salvage a narrative that turns into a bloody mess.