OK, maybe not the best costume I've seen.

As horror premises go, “What if Superman were a sociopath?” is among the best I’ve heard lately. “Brightburn,” a scruffy, nasty, low-budget effort produced by “Guardians of the Galaxy” director James Gunn and written by his brothers Brian and Mark, executes it fairly well, feeding on parental fears about the unpredictability of child-rearing but leaving some potentially fruitful layers of horror untouched.

In 2006, Kansas farm couple Kyle (David Denman) and Tori (Elizabeth Banks) Breyer adopted an alien baby whose space capsule landed near their home, just like Jonathan and Martha Kent did for baby Kal-El in Smallville. The Breyers hid the baby’s vessel and its other contents under a trapdoor in the barn; now, at the onset of puberty, Brandon (Jackson A. Dunn) starts to feel subconsciously drawn to it, his powers — and his true nature — starting to manifest. Imagine a 12-year-old boy with all the strength and ability of Superman but all the impulse control of, well, a 12-year-old boy. His behavior toward a classmate (Emmie Hunter) he has a crush on is highly disturbing, and would be even worse if he were a little further down puberty’s path and had a clearer idea of what he wanted.

The Breyers react to Brandon’s mood swings and violent outbursts with plausible defensiveness and concern. Kyle assumes it’s all hormones and takes the kid hunting so they can have a birds-and-bees talk. Tori is more attuned to the reality but, as a mother, more inclined to protect her baby at all costs. But as casualties mount in their small community (the town is called Brightburn), it becomes impossible to deny who’s responsible.

Directed by Gunn family pal David Yarovesky, the film is deliberately gory in a few places and low-grade suspenseful throughout, with committed performances by the three principals. (Dunn’s ice-cold Brandon is especially impressive.) It’s rough around the edges, with narrative choices that seem to have been made for budgetary reasons — Brandon’s aunt (Meredith Hagner) is also the school psychologist; the mother (Becky Wahlstrom) of the girl he likes is also the only waitress at the town’s only restaurant — but things like that give it a scrappy, under-the-radar feel. It mostly does the trick. That being said, there’s so much more that could have been done with this scenario, and the closing credits hint at an even more interesting sequel … a sequel I will definitely watch.

Crooked Marquee

B- (1 hr., 30 min.; R, some very graphic violence and gore, scattered harsh profanity, unsettling themes.)