Ma, a Blumhouse production directed by Tate Taylor (The Girl on the Train), is not a good film. It attempts to be, but everything it does is either too predictable, too hesitant, or too weakly developed to function. It has an excellent cast, including Octavia Spencer, Juliette Lewis, Allison Janney, and Luke Evans, but without an effective screenplay they have no way of making their roles work. There is potential here, but it is squandered.

Spencer plays Sue Ann Ellington, a veterinary technician in a small American town who ingratiates herself into a group of teenagers with the offer of free alcohol and a basement in which to hold parties. As he behaviour grows more obsessive and peculiar, one of the group – a girl named Maggie Thompson (Diana Silvers) – starts to suspect her of sinister motives.

There are numerous problems with how Ma is executed. For one thing it takes a long time to get going, with a leisurely narrative that regularly teases its audience without ever satisfying them. While there are twists and developments in the plot, they are crushingly obvious and easy to predict – and that makes the slow burn reveals even more frustrating. When the overt scenes of psychological horror finally emerge, they are either weirdly truncated or oddly recalcitrant. Moments of Saw-style violence are teased, but then curiously avoided. The end result is the worst kind of horror movie: the kind that pulls its punches. Taylor wants to attract a particular sort of viewer, but does not seem comfortable giving that viewer what they like.

Octavia Spencer is a sensational actor, and superficially the role of Sue Ann would seem a fantastic opportunity for her to play a much more uncomfortable character that her usual roles. While she valiantly plays with Sue Ann’s emotional instability and growing obsession, she is saddled with a character that makes no sense and is unevenly presented. It is as if the film cannot decide whose side it most wants to present, and instead gets messily trapped with an unsympathetic antagonist with a weirdly sympathetic back story. Both Luke Evans and Juliette Lewis are stuck with disapproving parent roles that honestly wastes their time and talent. It is even worse for Allison Janney, who gets a character with limited screen time and no clear reason to be in the story at all. The younger cast members, including Silvers, Corey Fogelmanis, and McKaley Miller, get few opportunities to distinguish themselves.

The bottom line? Ma seems to have been written as a deliberately trashy and knowing genre piece, high on self-awareness and camp. It seems to have been directed as serious drama, addressing the long-term ramifications of teenage bullying. The end results fail miserably at fulfilling either intention. It is as dreadful mess, but worse still it is a dull one.