"This is the story you get," Ma tells Jack during one of a number of gripping scenes in Room. Brie Larson, Ma, is trying to explain to her 5-year-old son, played by Jacob Tremblay, that there is an entire world beyond the walls of their dingy, dirty, confining home. Jack has never been outside. His only exposure to the sun is through a skylight in the roof. He has never been beyond the walls of "Room," a garden shed that has been turned in to a prison, complete with security keypad, by Old Nick, their creepy, domineering warden. His Ma is challenging him, and in turn all viewers of the film. This is the story you get -- a mother who loves her son and shares a deep bond with him, yet does so in horrifying, quick-turn-away circumstances.

Tension abounds in Room.

The captivity of its two principal characters is revealed slowly and methodically, but not unrelentingly. Accentuated by the physical proximity of everything to everything else -- by Jack's alternate sleeping quarters, a closet in which he can barely move around without some inadvertent bump or knock -- the garden shed feels hermetically sealed. It gets more grim each time you are exposed to Old Nick, played by Sean Bridgers, who is using that security keypad and sheer physicality to keep Ma locked away as a sexual slave. Jack is the product of this slavery, but, perhaps because of Ma's insistence or because he is an inconvenient reminder of his guilt, Old Nick hardly acknowledges his biological son.

There is of course natural tension derived purely from this situation. Why are we watching if Ma or Jack or both aren’t going to make a break for it at their first good opportunity? Surely, they’re going to get out of this place — this closet-sized college dorm bathroom — you begin to worry. This anxiety comes from the other tension in the film — the cognitive tension, the polarity even, of all these little mother-son moments that cut through the other tension.

Ma has done her best to construct an entire world for Jack. There are makeshift toys — a snake made out of eggshells — and drawings on the wall and cake for his birthday. This does not qualify as a good or healthy life. Jack knows little of the world and is certainly emotionally and intellectually stunted. But it is not an altogether terrible life for him either. This, in case you are wondering, is how a film that opens in a dilapidated garden shed and suffocates you there, right alongside its characters, for the better part of an hour can manage to still feel a little bit hopeful.

I don’t think it qualifies as a spoiler to reveal that Ma and Jack do eventually make it out of their cell. It is a momentary relief when they do, but the overwhelming complexity of dropping a nominally socialized 5-year-old in to the real world, replete with toys and animals and grandparents (Joan Allen and William H. Macy) and a mother just starting to recover from years of rape and mental and physical torture, brings the tension flooding back almost instantly.

“You’re going to love it,” Ma tells Jack when describing the real world just on the other side of the walls of Room. Larson’s voice sounds almost tearful as she says it. She is speaking with conviction and with more than a twinge of melancholy. The world is beautiful and complex and terrible. There is much for Jack to learn, good and bad. He has friends to make and legos to play with and a grandfather that won’t accept him and a mother that, with freedom, is searching for resolve, meaning, purpose, and wading through an ocean of depression.

He has his own person to become, and here is the final bit of tension in the film. Room is filled with little moments between Larson and Tremblay that are worth treasuring. Director Lenny Abrahamson, toying with camera angles and perspective and voiceover from Tremblay, makes these moments at once wonderful and fleeting.

The very best of these is in the hospital room they are sharing immediately after being rescued. The room is warm and bright and white and offers a bird’s-eye view of Cleveland. This room is everything Room is not. You savor the peace and quiet with Larson. You feel refreshed when she takes her first shower in the better part of a decade and smile as she explains to Jack exactly what a shower is and splashes water in his face.

She can only keep the wolves at bay for so long. Soon enough, a doctor enters the room and begins to counsel her on what medical and psychological treatment Jack might need. Not long after her parents barge in. Soon after that, she’ll learn her parents have divorced in the intervening years. Reality — impossibly bleak or unbearably mundane — is always on the other side of that door.

Room seems to advocate holding tightly to all those little moments. They are what get us all through, the film suggests. This is a small, claustrophobic story about a very, very large thing. It makes a true star of Brie Larson. It is terrific in just about every way. This is the story you get. Appreciate every moment of it.