Briony accidentally spies her older sister Celia (Keira Knightley) making love to the housekeeper's son, Robbie Turner (James McAvoy) in the family library " their passion erupting, appropriately, in a room full of words. Briony, who has had a crush on Robbie, concludes that he is a 'sex maniac" and has attacked her sister. So later, when Briony's cousin Lola (Juno Temple) really is violated, Briony tells authorities that she has seen Robbie commit the rape. Robbie and Celia vigorously protest his innocence, and suspicion is directed toward other characters, but Briony's testimony proves unshakeable, and Robbie is imprisoned for four years and released only to serve with the British Army in World War II prior to and during its desperate retreat to Dunkirk.
In part, Atonement addresses the attendant prejudice of the British class system. Robbie is Oxford educated and on his way to medical school when Briony accuses him, and the picture leaves little doubt that a child's word would have counted rather less against a young man judged Celia's social equal. Though the novel is at greater pains than the movie in this regard, the gardener's son is a suspect for reasons of class alone.
Faithfully following the structure of McEwan's book, Atonement is also determined to provide some historical qualification to 'the good war" and 'greatest generation" shorthand that has become so popular in recent years. The filmmakers aren't concerned with the larger questions of whether World War II had to be fought, but they are interested in exploding those myths that have bloomed to suggest that all actions by the allies were heroic and all decisions by allied leadership wise. Though several hundred thousand soldiers were finally evacuated from the beaches at Dunkirk, Atonement demands that we see this critical event not as a triumph but as a disaster that can be celebrated only for having avoided being even worse. The British were reduced to destroying their own weaponry and transport (including untold numbers of horses) so that they would not fall into the hands of the enemy. Moreover, indecisive leadership proved almost critically indifferent to the welfare of its own men. The scenes at Dunkirk here depict a hell on earth, something that will evoke a surreal nightmare as if out of Dante's Inferno.
Ultimately, though, Atonement is a reflection on the power of artistic rendition, the power of the artist to make the world a better place. As Briony grows up, she realizes the horror she has done and in several ways tries to use her power over words to atone for the grievous sin of her youth. But the real world does not submit as easily as the fictional to an artist's rewrite. Upon reflection, a novelist or screenwriter may decide to solve a mystery another way as opposed to the one first attempted or may opt not to kill a character after all. Or the author might choose to afford forgiveness to someone, in contrast to the human world where divine forgiveness rather than that of our fellows is the best we might hope for. For in the real world, once words are spoken, they cannot be sucked back into silence. And however much we wish they might, events will yield to no eradicating fluid once they have been written in the indelible ink of time.