Atonement is about a moment in a life. Briony Tallis, a thirteen year old girl, tells the police that her sister's boyfriend is a rapist who has raped another young girl. Briony's statement comes out of facts we learn later on that it would be unfair to ruin the film through using, but also because she misunderstands the situation. Seeing her sister's boyfriend and her sister together in two incidents she interprets her sister's boyfriend as a sexual predator- and her sister as his victim. When she picks up a note from the boyfriend to the sister which mentions kissing her sister's soft wet oriface, she presumes that this is yet another part of the boyfriend's aggressive strategy against the sister. Consequently she out of a mixture of motives and a mixture of understandings, decides to inform the police that the rapist of her cousin is the boyfriend, Robbie. She tells them that despite the fact that later on she recalls having seen another man doing the crime, despite the fact that it may be that she actually knows at the time that it was another man.
Over and above that though the film keeps coming back to this issue- which is a central moral and psychological issue as well- if you do something terrible and what you do has consequences you can see in terms of dead bodies and dead lives, then have you by chronicling it or by living your life as a homage to it attoned for it. What Briony does destroys three lives- it sends a victim off to marriage with her rapist, sends a girl off to leave her family behind and lose her boyfriend to prison and sends that man off to war and prison, losing him all the fruits of adulthood- all his attempts to become a doctor and a good husband. The central issue to the film and to McEwan's book is this- its the question of what price one can pay to have that moment that Briony implicates her brother in law in rape back again. For Briony the suffering lasts right until the moment, which we see, in which she effectively dies. In some ways J.S. Mill's judgement that lifelong prison is worse than death comes back to one- the others all get real deaths, for Briony there is the lifelong prison of her guilt- a prison that will never let her escape the moment when she was thirteen and falsely accused an innocent man.
Everyone suffers from that decision taken by a thirteen year old, and that decision reverbrates through these lives, doubling and redoubling, having more and more impact as the years go by and Briony's guilt increases and her inactivity increases with it.