Brie Larson, Joan Allen in 'Room'

THINK OF "Room" as "Boyhood" as it might have been made by David Fincher. The two movies even have the same defining shot: a boy on his back, looking at the sky, which represents an blue expanse of infinite possibility.

Except that in "Room," when a boy of 5 looks at the sky, he's essentially seeing it for the first time. And he's in the back of a truck driven by a man who's held him captive his entire life in a shed, a cramped space the boy shares with his abducted mother (Brie Larson), who trades sex for meager supplies of food.

Larson is getting Oscar buzz. This is a testament to her performance and to the strangely heroic nature of her role: In the shed, Ma (as son Jack calls her) labors imaginatively and tirelessly to make this small space a seemingly safe, nurturing place, a universe whose borders are their shared imagination.

But we also see Ma biding time. She's been plotting escape since she was snatched off the street at 17; she still bears the scars for her last attempt.

When Jack (Jacob Tremblay) gets sick, Ma sees her chance - borrowing an idea from one of the boy's favorite stories (hint: it's Dumas). She gives her child a chance at freedom, but it means being honest about the outside world.

The psychology of the movie (the screenplay was written by Emma Donoghue, adapted from her best-selling novel) is interesting. Ma creates a viable world for her son, then must destroy it in order to prepare him for the reality of the world he will encounter if he's to escape, if he's to live a free and full life. You can read that, of course, as a heightened and horrific compression of parenthood in general.

Filmmakers enjoy the challenge of these claustrophobic spaces - "Locke" last year, and "All Is Lost" the year before. Audiences, not so much.

So it's worth noting, and not revealing too much, that "Room" leaves the shed, and mother and son end up in the home of Ma's mother (Joan Allen).

Again, the psychology is interesting: How does a woman who's devoted all of her mental energy to a life-sustaining illusion adjust to real life? Who is she when her all-consuming task is over?

It's a problem for "Room" that Larson, who commands our interest, suddenly disappears for portions of the movie's final third. The movie is wedded to the boy's perspective throughout, and we stay with him at his grandmother's home while Ma gets treatment elsewhere.

But we miss Larson's hold on us, and we sense there another movie to be made of Ma/Larson learning to live again.

We don't get to see it, but the movie we have is good enough.

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