Andrés Muschietti’s debut feature “Mama” is hardly the first horror movie to riff on the idea that other people’s children are little strangers, mysterious beings whose motives are impossible to fathom. But “Mama” mines that idea with considerable grace and more than a few chilly glissandos. At least until the third act (the point where, admittedly, so many ambitiously refined horror movies tend to fall apart), the picture skims along on a pleasing current of unease – and it also features a surprisingly relaxed performance from ubiquitous superactress Jessica Chastain.

“Mama,” which Muschietti adapted from a short film he made in 2008, opens in the aftermath of a single grim act, just as a second and even more horrific one is about to take place. A clearly distraught father (played by Nikolaj Coster-Waldau, from “Game of Thrones”) herds his two young daughters, aged 3 and 1, into the car and begins speeding along a curvy, treacherous mountain highway. The car swerves off the road and tumbles down a hill; father and daughters survive and make their way toward an abandoned house deep in the woods, a ramshackle cabin appointed with midcentury modern furnishings that clearly haven’t seen use since the midcentury.

The father, we learn, has gone berserk after taking a financial hit. He’s already murdered the girls’ mother (the act is referenced but never seen) and now intends to take his daughters’ lives and then his own. But a spindly, shadowy figure -- presumably the inhabitant of this foreboding little cabin -- intervenes, cracking his spine in a ghostly, half-glimpsed ju-jitsu move.

Jump forward five years, and the girls’ whereabouts are finally discovered by a rather inept search party employed by their uncle, Lucas (also played by Coster-Waldau), who has been desperate to find them. Having survived all that time with no human contact, the girls are like little forest creatures, somewhere between the wolf boy of François Truffaut’s “The Wild Child” and the preverbal Helen Keller of “The Miracle Worker.” Naturally, Lucas vows to adopt them, and while his wanna-be rocker girlfriend Annabel (Chastain, in a sleek, dark Joan Jett bob) has her doubts about being a mom, she stands by her man.

The older of the two girls, Victoria (Megan Charpentier), is somewhat redeemable, remembering enough about her old life to adjust to the comfy suburban upbringing that Annabelle and Lucas hope to provide for them. But Lilly (Isabelle Nélisse) is a spidery little creeper, and she can’t shake her attachment to a remnant of their woodland life, a possibly illusory figure the two sisters refer to as “Mama.”

Mama has provided the tots with some scary-looking dolls made of twigs, just one example of the atmospheric touches Muschietti is so skillful at layering. (The script is by Muschietti, Neil Cross and Barbara Muschietti, and the movie was produced by Guillermo del Toro – it bears earmarks of his intelligent, half-dreamy, half-spooky view of childhood.) “Mama” is one of those pictures that holds you aloft on its vaporous mood of dread – the occasional silliness of the plot mechanics don’t matter so much. (Also, animal lovers will also want to heed this important public service announcement: Nothing bad happens to the dog.) The picture falters only in the last third, when the visuals become far too direct and obvious – the things you don’t see are always so much scarier than the things you do, and “Mama” makes the mistake of spelling things out that were better left unseen.

To its credit, though, the movie has enough conviction to go for a not-so-happy ending. And even if, at this point, you’re a little sick of Golden Globe winner and all-around critics’ prize hogger Chastain, she’s refreshingly casual and low-key in “Mama,” believable, even, as a tattooed bassist in an amateur (and pretty lousy) rock band. Annabel’s feelings for her two strange young charges – which shift from resigned ambivalence to unsentimental protectiveness – play out believably and gracefully, in Chastain’s body language and in her eyes.

Maybe Chastain is so good here because she doesn’t clamp down on the role: There’s nothing actressy or overstudied about what she does, no evidence of a young performer striving to prove herself. In fact, this seemingly throwaway performance has more contours than the one she gives in Kathryn Bigelow’s generally stunning “Zero Dark Thirty”: With the exception of the last scene in that movie, her characterization of an ambitious young CIA agent suffers from a peculiar emotional vagueness. In “Mama,” a picture that isn’t going to win Chastain any awards, she’s both more specific and subtler. This is a picture about how a mother’s love can be nourishing at its best but suffocating in the extreme. Chastain embraces that idea wholly, without squeezing the life out of it.

Grade: B-