Emma Donoghue’s novel—about the prolonged, claustrophobic abuse of a five-year old boy and his mother, as told by the tyke himself, followed by its traumatic aftermath—seems an unlikely tome for screen adaptation. But in the event, as adapted by the author herself and directed with sensitivity and skill by Lenny Abrahamson, “Room” proves a compelling film.

The picture falls into two distinct parts. In the first, Joy (Brie Larson) and Jack (Jacob Tremblay) pass their days and nights in a small shack equipped with sink, toilet, microwave and a skylight but nothing else beside the bare essentials. The only door is equipped with a combination lock with the numbers known only to Old Nick (Sean Bridgers), a bearded man who comes at night to bring food, but mostly to have sex with Joy. During those nocturnal sessions Jack, who’s just celebrated his fifth birthday, peers out at them from his bed inside a closet.

Much of this initial section of the film is devoted to the close, loving relationship between mother and child, punctuated by an occasional childish tantrum. But after sheltering Jack for half a decade, maintaining the illusion that their cubicle is all there is to the world, Joy tells him the truth: she was abducted by Nick seven years earlier and kept a prisoner in their “home,” his converted backyard garden shed. Jack finds it difficult to comprehend the implications of the revelation—after all, he’s never experienced the outside world—but eventually he accepts the truth, and she enlists him in an escape plan involving his pretending to fall seriously ill. That plan metastasizes into something far scarier, but it ultimately succeeds.

That leads to be film’s second chapter—in which Joy and Jack struggle to come to terms with the reality beyond the room that has been their whole world for so long. It’s a difficult process for Jack, for whom everything is entirely new and not a little frightening, and the film follows his coming out of his shell with care and feeling. In many respects, however, their liberation from confinement proves even more traumatic for Joy. She’s not only aware of what she’s missed and suffered in the meantime—which ultimately causes a psychological crisis—but has to deal with the fact that though her mother (Joan Allen) is fully supportive, she’s divorced and with a new partner, the likable Leo (Tom McCamus), who becomes a surrogate grandpa to the boy after Joy’s father (William H. Macy)—in a sequence that’s brutal in its abruptness—can’t bear even to look at the person who reminds him of his daughter’s torment. (It’s a plot thread that might profitably have been examined later on, but isn’t.) There are a few narrative elements that don’t quite work–an ill-advised television interview comes off as a stiff, clumsy send-up of “tell-it-all” programming. Still, the film ends on a note of hopefulness, with both mother and son ultimately confronting—and rising above—their ordeal.

Without intending any pun, “Room” is a small, confined film in physical terms, but emotionally it’s expansive and resonant. That’s due to Donoghue’s trenchant but understated screenplay, which keeps a sharp focus on the personal and the specific, and from Abrahamson’s sense of control and reserve; together they convey the power of the narrative without sensationalizing it in the style of a lurid telefilm.

But the impact of “Room” largely derives from the performances, especially the two leads. Larson has made a vivid impression in smaller roles a number of independent productions—“The Spectacular Now,” “Digging for Fire”—and a major one in “Short Term 12.” She surpasses that work here, however, with a heartbreaking depiction of parenthood under siege. Even more of a revelation is young Tremblay, who—presumably with strong guidance from Abrahamson—catches the changes in Jack’s personality with amazing insight. Macy is predictably forceful even if his character remains a bit of an enigma, and both Allen and McCamus exude gentle but firm humaneness. Bridgers, on the other hand, quietly completes the trilogy of scumbags that began with “The Best of Me” and continued with “Dark Places,” though here he creates a loathsome character in a more restrained manner than previously. The film is hardly innovative technically, but Abrahamson and cinematographer Danny Cohen make surprisingly good use of the anamorphic frame in the first, closed-in segment of the picture, and editor Nathan Nugent shows particular skill in Jack’s “escape” sequence.

Abrahamson seems to thrive on concepts that threaten disaster—a lead character who constantly wears a ludicrous mask in “Frank,” here a first act that traps two characters in a tiny space. But in both cases he overcomes what appears to be an insurmountable obstacle to achieve a result that almost makes you forget the limitations he’s embraced. This is a strong, remarkably acted tale of the bond between mother and child tested by the most extreme circumstances.