“Mama” breaks a lot of horror movie rules, right off the proverbial bat.

It gives us a long back-story opening, and brings up much more back story as the tale progresses.

It over-explains. It reveals its supernatural menace, not just in glimpses, but full on, and early on. There’s never any doubt that this might be all in somebody’s head.

But “Mama” is a reminder that the best chills don’t involve chainsaws, blood and guts. Horror is a product of empathy - in this case, fearing for the safety of small children and the reluctant twenty-something rock musician (Jessica Chastain) stuck with caring for them.

A prologue tells us of a tragedy. A distraught father (Nikolaj Coster-Waldau) flees financial scandal by shooting people, grabbing his children and fleeing into the snowy mountains of Virginia. They crash, he drags the innocent little girls to a remote cabin, and just as he is about to finish his horror something happens to him.

Cut to five years later, and searchers finally find the girls. They’re feral, non-verbal, skittering around on all fours like rats. Their artist Uncle Lucas (also Coster-Waldau) is ready to take them in. His bass-playing girlfriend, Annabel (Chastain) is not.

“Don’t call me that,” she says with a smile when Victoria (Megan Charpentier) calls her “Mom.” She’s not. “This isn’t my job,” she tells Lucas.

But thanks to financial arrangements made by the conniving psychotherapist (Daniel Kash) who sees glory in their case, the D.C. couple moves to a free house in Richmond and tries to bring the girls - Lilly (Isabelle Nélisse) doesn’t speak, but only gurgles, grunts, eats cherries and sleeps with tree limbs - back into the human race.

Thanks to whatever kept them alive for five years in the woods, that’s not going to be easy.

Producer Guillermo del Toro (“Pan’s Labyrinth”) must have had a hand in the production values here, which are state of the art. But what makes “Mama” work are the performances co-writer/ director Andres Muschietti got from the little girls, who are open-faced marvels, conflicted about where their loyalties lie - with “Don’t call me Mom,” or with “Mama.”

And Chastain, far from slumming in a horror film just as she’s fighting for that “Zero Dark Thirty” Oscar, adds another gold star to her resume. Annabel is unhappy, ill-equipped for parenting, stand-offish. Chastain makes her sexy, immature and yet somehow sympathetic.

Horror is all about the short-circuit that the screen’s technical manipulations - music, editing - cause in our brain, so this isn’t high art. But “Mama” is easily the most moving, most chilling ghost story since “Insidious,” an emotional tale efficiently, and affectingly told.