Mama is, for the most part, a nifty little ghost story that's at times unpredictable and happily, not your average horror flick that's totally dependent on false scares and stops. There are some of those in here, yes, but there's also an emotional undercurrent that's genuinely affecting, until the disappointing and bombastic ending practically ruins it.
It starts out as a unique and chilling mystery, with two little girls whose father (Game of Thrones star Nikolaj Coster-Waldau, who later plays his own twin brother) kills their mother and crashes their car in the woods, hiding them out in a cabin and intending to kill them too until a mysterious entity stops him. Five years later, the girls are found, still young (8 and 5) and nearly feral, having been raised in the wilderness by an invisible being only they can see. Now they're brought back to civilization and taken in by their uncle and his girlfriend Annabel, played by Jessica Chastain, a tattooed and somewhat Goth rocker who doesn't like or want kids in the first place. Of course, their friend "Mama" comes with them, and the spookiness continues in the girls' new domestic setting.
There are further plot complications, as a court appointed psychologist tries to drag the truth out of the older girl, Victoria, but what works best in the movie is Chastain and her evolving relationship with the girls and eventual rivalry with Mama over guardianship of them. Annabel at first comes across as abrupt and nearly mean to these poor kids, but Chastain is such a good actress that she conveys Annabel's changing emotions so clearly and gradually that we believe it as we see it happen, and we start to root for the logical ending, which is for Annabel to save the kids from Mama's firm, ghostly grip on them.
Unfortunately, despite a few good scares and some effective creeping atmosphere as Annabel tries to figure out what's haunting them, the movie gets way too caught up in the plot, which becomes unnecessarily complicated and starts to lose its way with certain characters like Uncle Luke (Coster-Waldau). He serves a very real purpose in setting up the story, as the brother who wants his nieces, but after a while it becomes obvious there's no more role for him to play, and so the movie simply gets rid of him by putting him in a coma for what should have been the remainder of the film. When Coster-Waldau returns to the story the character is now completely out of place and is part of the overstuffed climax that takes things to a dark and unsatisfying turn that leaves you highly disappointed and almost angry at what passes for the finale.
Despite the last 15 minutes however, I did like most of the film and the story itself kept me interested, entertained and contained enough good scares to recommend itself to horror fans (Mama herself looks extremely creepy and is a very well designed CG creation). But, man, that ending. Sometimes knowing how to finish a film and take it to its logical conclusion is all you need to do, and trying to add unnecessary last minute twists to avoid cliches is going a bit too far when you've done more than enough already in that department. I suppose I sound a bit like I'm saying give the people what they want, but in this case, yes, that would have been wise.