Mesmerizing performances in harrowing mother-son tragedy ‘Room’

Starring Brie Larson, Joan Allen and William H. Macy. Directed by Lenny Abrahamson.

Rated R for language. Check listings for theaters. 1 hour, 58 minutes.

Bottom line: A splendidly acted film with fascinating qualities

Much of the effectiveness of “Room” — and it’s very effective — depends on not knowing every narrative turnabout in advance. We’ll be as clear as we can while hinting around at a few things regarding director Lenny Abrahamson’s splendidly acted, if ever-so-slightly dodgy, film version of the 2010 Emma Donoghue novel.

The premise is simple and brutally confining. A young woman known only as “Ma,” played by an unerringly true Brie Larson, lives with her newly 5-year-old son, Jack (Jacob Tremblay, astonishingly just as good as Larson). She is a valiant and rigorous mother: playful, inventive, a believer in routines, good oral hygiene and a limited amount of TV-watching time.

Ordinary people living ordinary lives, from the sound of it. Yet theirs are being lived inside a 10-by-10-foot garden shed, equipped with a tub, a sink, a skylight, a bed, a closet and a toaster oven. “Ma,” we learn early on in “Room,” was abducted as a high schooler by a man known only as Old Nick (Sean Bridgers). Raped systematically over several years, the prisoner gave birth to a boy who does not know the identity of his father.

For a time, we watch in a state of dread and wonder. Jack’s notions of reality and fantasy are entirely his own. The universe, as far as he knows, belongs only to the place he calls Room. Once Ma fashions an escape plan for Jack, we’re hit, abruptly, with life outside Room, which is hugely disorienting for both mother and son.

Joan Allen is superb as Ma’s mother, whose mind is a tangle of guilt, gratitude, anger and longing for peace for her daughter and grandson. William H. Macy, as the Larson character’s father, offers a portrait in inchoate grief; Tom McCamus is drolly understated as Ma’s empathetic father figure, her mother’s new man.

The second half of “Room” is the divisive half. I think the second half is no less intriguing than the first, though certain elements of both script and direction are worth debating. Some are small details, but important ones. Why, for example, did director Abrahamson (whose previous feature was the lovely, bizarre black comedy “Frank”) stage the crucial sequence with Jack in the back of a pickup truck so clumsily, risking logic and credibility?

The larger issue, I think, is one of finding a tone for “Room” that is honest and love-filled yet not sappy. If Abrahamson’s film had another 10 minutes to it, those 10 minutes would’ve fruitfully been used to finesse Ma’s hitting bottom in the later scenes, before she and Jack come back up again. As is, the recovery feels too easy, too upbeat, for what Donoghue’s characters have endured.

The parent/child relationship at the movie’s core is endlessly fascinating. And in the key scene between Larson and Allen, when the shock gives way to old recriminations, the movie reminds us that stories such as these can never end in the middle.