The first hour of Atonement, set in a pre-war English country house, is faultless: a pungent stew of pleasure and dread, shrill suspicions and pouting revenge.

Briony Tallis (Saoirse Ronan), a pale-haired child looking almost spectral in her shapeless white dress, has written a play called The Trials of Arabella. She's preparing for the evening's performance when her older sister Cecilia (Keira Knightley) has a sexually charged altercation with her childhood friend and social inferior Robbie (James McAvoy). A shard of pottery falls into a fountain, and Cecilia strips down to her underwear to retrieve it, humiliating Robbie—and stupefying her baby sister, who watches the silent scene from a window, unobserved.

As Cecilia and Robbie's animal affair staggers forward, Briony continues intercepting half-understood crumbs of information. Faced with evidence of mutual provocation, she assigns all the desire to Robbie, concluding he's a "maniac." So when a cousin is raped and Briony catches a glimpse of the perpetrator, she knows—she convinces herself—that she has identified the man responsible. But she has not, and Robbie and Cecilia will suffer for her misplaced conviction.

The film's casting is brilliant, the production design impeccable, the point-of-view switchbacks beautifully turned. Sloughing off the novel's pretentious narration—a pastiche of Mrs. Dalloway that turns even less agreeable once author Ian McEwan blames a grown-up Briony for writing it—the film nonetheless bows to his conceit by weaving the sounds of a typewriter into the score.

The second half of the film is disappointing, relative to the first, but it's not entirely wrongheaded. While you're watching, a haze of outrage at Briony's actions mixed with a drop of sympathy for her immature passion sweeps you right through to the end. And doesn't it seem right that the world after her fall would be made of weaker, paler stuff than the perfection that preceded it?