"It's like Unbreakable Kimmy Schmidt but done seriously" was the common pre-TIFF refrain about Lenny Abrahamson's adaptation of Emma Donohue's Booker-nominated novel. But that's giving this tale of a kidnapped woman (Brie Larson) and her son (Jacob Tremblay) trying to escape their personal prison seriously short shrift. The way the Frank director continually ratchets up tension is almost literally breathtaking; there's a sequence at the halfway point that's so unbearably tense that the producers may want to keep paramedics on-hand in theaters. And though the last act starts to tiptoe into Lifetime-channel territory, the performances of Larson and the eight-year-old Tremblay — where the hell did they find this kid? — more than make up for any minor maudlin missteps.