The mama in Mama is pretty freaky looking—-like a part ghostly, part human-shaped being made up of mildew, Medusa hair, and black cobwebs who crawls up walls and moves as if she has no bones. (Which, duh, I suppose she doesn’t.)

The problem with her (it?) is that the more you see of mama, the less scary she gets—-to the point of being kinda laughable. And as her fear factor tanks, so goes the film, culminating in a truly lame ending that negates any genuine spookiness that came before it.

The story, directed by freshman Andres Muschietti, starts with a father (Nikolaj Coster-Waldau) taking his two young daughters into the woods after killing their mother. He’s about to kill them, too, when something witchly dispatches with him. Five years later, the girls, Victoria (Megan Charpentier) and Lilly (Isabelle Nelisse), are found, feral and spider-walking. Their uncle, Lucas (Coster-Waldau again), and his girlfriend, Annabel (Jessica Chastain, all black-haired, gothed out, and ultimately wasted), are given custody and, it’s implied, a “good luck with that” from the authorities and analysts who can’t seem to reintegrate the girls into the civilized world.

The couple eventually bonds with the kids, of course, though Annabel’s then left on her own when Lucas is hospitalized after a guess-who-did-that fall down the stairs. Things that go bump in the night get even bumpier, with the girls first just talking to and playing with someone unseen and saying “mama” a lot. But mama’s a jealous type, and suddenly not so shy.

Mama, to Muschietti’s credit, relies on cheap jumps only sparingly, and for much of the film, there’s a real creepiness here; the feel, between mama’s looks and the tone in general, is very Insidious. (But from now on, let’s put a moratorium on the smoky-black-blob thing, OK?) Overall, however, it doesn’t live up to its “presented by Guillermo del Toro” pedigree. (Though it’s admittedly miles above Don’t Be Afraid of the Dark.)

Kids plus ghosts is a pretty reliable horror formula. The trick is getting the balance of showing versus suggesting just right, and Mama reveals way too much to keep the shivers going.