‘Room’ finds Brie Larson doing huge things in a tiny space

There’s acting, there’s ACTING and then there’s what Brie Larson and a nine-year-old named Jacob Tremblay achieve in the new movie “Room,” which is something closer to a small miracle.

The film is about the special, intractable bond between a mother (Larson) and her 5-year-old son (Tremblay), amplified immeasurably by the fact that they are both the prisoner of an abuser, who keeps them locked in a tiny shed in his backyard.

Lenny Abrahmson directs Emma Donoghue’s adaptation of her novel, and the action is constrained to that prison for nearly the first hour.

So it lives largely in the head of the 5-year-old, named Jack, who has never experienced the world outside the tiny space and thus finds the plight of every child, seeking to process and understand what they are seeing and experiencing, warped by his circumstances. Nonetheless, his Ma is singularly devoted to him and their connection, as it plays out in the few feet between their bed, their closet and their small kitchen, has a quality that is at once otherworldly and inherently familiar.

That this story has become a movie at all is hard to believe; this had to have been a challenging adaptation, even for the novelist. It’s constrained, the tone vacillates between flights of whimsy and harrowing realism; between tender moments between mother and child and grim psychological torture. Abrahmson last made the movie “Frank,” in which Michael Fassbender played a singer who permanently sports a giant papier-mâché head, so he knows something of difficult material. He’s done an admirable job of opening it up and engorging the audience into the shared head space of mom and child.

But it’s Larson and Tremblay, so authentic and natural that you feel like each is physically conjoined to the other, that take a curiosity and transform it into a considerable emotional experience. There are few more resonant examples of two actors seeming to belong together than this.

Their characters experience a significant trauma over the course of “Room,” but they have each other, and they make us believe it will be OK.