“Mama” breaks a lot of horror movie rules. It opens with a long back-story, It over explains. It reveals its supernatural menace 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 and blood. Horror is a product of empathy — in this case, fearing for the safety of small children and the reluctant twenty-something rocker (Jessica Chastain) 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 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 find the girls. They’re feral, non-verbal, skittering around on all fours. Their artist Uncle Lucas (also Coster-Waldau) is ready to take them in. His bass-playing girlfriend, Annabel (Chastain) is not.

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

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.

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.

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.