Starring Brie Larson. Rated PG.

With 2014’s Frank, director Lenny Abrahamson took Michael Fassbender and made him spend an entire film with his million-dollar jawline concealed inside a papier-mâché head. Based on that caper, Abrahamson is either insane or a genius—probably the latter, since Frank was such a blast—but the Irish filmmaker prevails against an even greater set of risks with Room, turning Emma Donoghue’s best-selling novel into a devastatingly effective piece of entertainment.

The front section takes place in the room of the title, actually a reinforced suburban shed where Ma (Brie Larson) has been imprisoned for seven years by her captor, Old Nick (Sean Bridgers). Raised inside “Room”, five-year-old Jack (Vancouverite Jacob Tremblay—incredible) has no concept of the world beyond a single skylight. Abrahamson imbues his miniature universe with a child’s phantasmagoria of meaning, while sidelong glimpses of Ma reveal a fraying caregiver relentlessly protecting the emotional health of her child.

This is already stunning stuff, but Room raises the ante with its second half, following an escape—no spoiler here—that constitutes the 10 most stressful minutes any viewer will be asked to endure in a cinema this year. Once free, Jack and Ma are met with anything but the relief she dreamed of, even inside the sensitive embrace of the family home with Grandma Nancy (Joan Allen).

In the end, Room is really about the impossible choices we face as parents, where the right decisions are just as easily the wrong ones and the stakes are too great to contemplate without imperilling our sanity. How Abrahamson and his uniformly amazing cast achieve this almost imperceptible subtext deserves to be seen.

If Room is also faintly manipulative—Stephen Rennicks’s score too often underlines the obvious—a key scene involving a heartless, if incisive, journalist played by Wendy Crewson is rife with the kind of painful, grown-up ambiguity that’d make Steven Spielberg’s head pop.