Atonement is about a tattletale, a precocious, imaginative kid who says the darnedest thing: “I saw him. I saw him with my own eyes.” But this 13-year-old girl, Briony, isn’t merely singing on her big brother for taking a puff on his friend’s cigarette. She’s flinging a rape accusation at the housekeeper’s choirboy son, Robbie, who has ambitions bigger than his humble background. Robbie is also the lover of Briony’s sister, and he’s just declared his feelings for her. Briony likes the young man, too. But she did not see him commit the crime with her own eyes.

Based on Ian McEwan’s 2001 novel, Atonement opens in 1935 England, and the picture of privilege and dark polished wood it offers in the first chapter is immediately captivating. Director Joe Wright, who nailed the look and feel of Pride & Prejudice (if nothing else) in his 2005 adaptation, introduces us to Briony (Saoirse Ronan) with the clack of a typewriter providing the soundtrack as she finishes her first play. Briony stares intently—we’ll soon learn that’s pretty much her standard expression—as she types out “The End,” then speed-walks through her halls of her glorious home with the ramrod posture of a headmistress—in contrast to the femininity of her loose white dress—to show her mother the manuscript. Briony then consults with her older sister, Cecilia (Keira Knightley), on the lawn (impossible expanse, stunning green) before returning inside (high ceilings, creamy, flower-patterned décor that’s repeated in the characters’ clothes) to cast her cousins, including hopeless 9-year-old twin boys who, though bratty, still use words like “amenable.” Ahhhh, you think.

Briony is tart-tongued and beyond her years, but she doesn’t quite understand the world as thoroughly as she believes she does. She doesn’t know, for instance, what she’s spying through a window when she watches an encounter between Cecilia and Robbie (James McAvoy), which ends with Cecilia diving into and then emerging from a fountain, her wet, light-colored dress leaving little to the imagination. It’s hard to tell if the two are being antagonistic. But Briony knows a bad word when she sees it: When Robbie mistakenly has the girl deliver the wrong apology letter to Cecilia—a jokey confessional, dashed off during a bout of writer’s block—Briony deduces from his lurid vocabulary that he’s a “sex maniac.” Cecilia, though shocked at the letter herself, isn’t quite as concerned, though, and the couple end up consummating their crush quite spontaneously in the family library, Cecilia pinned against the wall. Which, yes, Briony also witnesses. So when, after dinner, the sisters’ 15-year-old cousin, Lola (Juno Temple), is attacked on the dark grounds, naturally Briony points the finger at Robbie. Lola goes along.

Though there’s already drama aplenty by then—which Wright nicely highlights by employing tiny time shifts to repeat scenes from different perspectives—Atonement is really about what happens after the accusation. Robbie gets sent off to jail and then war, while Cecilia becomes a nurse. They see each other sporadically, but the magic of that night in the library is lost, along with any sense of their previously genteel life. Robbie didn’t do it, and Briony—whether immediately or after a time, it’s not clear—knows it but is too cowardly to speak up. Instead, she, too, becomes a nurse when she turns 18, scrubbing bedpans and sitting with dying soldiers as acts of penance while she solicits the couple’s forgiveness through letters.

This is Briony’s story, so it’s not surprising that Atonement lags during its middle chapters, when the focus shifts to Robbie’s ordeals. The scenes are intended to drive home how horribly the girl’s mistake cost him, but paradoxically, they’re among the film’s dullest despite being visually inventive. The script places him, for example, at the Dunkirk evacuation of 1940, and Wright takes a long, single pan of a beach teeming with half-crazy soldiers, ships with tattered sails, and general chaos to capture the terrors of Robbie’s new reality. But the shift is too sudden, and we’ve had little time to get to know Robbie by this point. As the camera whirls on and on and on, you’re more likely to yawn instead of cry. Cecilia, too, is mostly an afterthought, and though Knightley’s wrenlike features make her acceptable decoration for a period piece, her calls to “act” aren’t quite as believable. (The fault, though, likely falls more heavily on Wright’s shoulders, who directed Knightley’s Elizabeth Bennett to appallingly uncharacteristic giggliness in Pride & Prejudice.)

Briony is played by three actresses—Ronan at 13, Romola Garai at 18, and Vanessa Redgrave as a senior—and all are enchanting. (Ronan in particular has a remarkably self-possessed, haunted quality that would be impressive for any newcomer, not to mention one her age.) Garai’s contribution is thankless, though, because of the wan scripting of the film’s middle section: Composer Dario Marianelli offers the urgent music of a thriller, but Briony’s guilt—expressed mainly by a camera trained on Garai’s sullen face—just isn’t all that gripping. Redgrave’s appearance, though, reawakens the film, both because of the actress’ exquisite ability with subtle emotion and a plot turn that reframes much of what you’ve just watched. For both the character and the filmmakers, atonement is achieved.