Cecilia (Keira Knightley) has developed feelings for Robbie (James McCoy), a man from a considerably lower social standing than her family and their grand estate. They are in love, and he is trying to adapt to their wealthy lifestyle by attending Cambridge, funded by them, with the intention of becoming a doctor. While this is happening, their actions are being intensely observed by her younger sister, Briony (Saoirse Ronan), after she misunderstands an event and reads a private letter. Briony becomes obsessed with the couple, then sets out to ruin them, fabricating a story that not only accomplishes this but leads to Robbie’s unjust imprisonment for a serious crime. The outbreak of war provides an opportunity for the lovers to re-evaluate their relationship, while a now older Briony (played by Romola Garai), who has drifted apart from her sister, experiences a significant moral struggle and goes to London to work as a nurse and confront the consequences of her actions from five years prior. This is a rich and intricate film; McAvoy performs exceptionally well portraying the honest and decent young man entangled in a network of deception and jealousy. Knightley’s performance isn't quite as strong, but she still contributes adequately until the truth emerges before an unavoidable tragedy unfolds. The story explores the impact of a falsehood, but it also examines people’s capacity for love, forgiveness, and judgment. Allegiance can be shallow, but remorse endures; and that endurance may not last as long as one might wish. Dario Marianelli has composed a superb score to accompany this narrative, and the writing combined with Joe Wright's understated direction prevents the film from becoming overly dramatic or sentimental as the ending approaches, featuring Vanessa Redgrave in a suitably concluding confession. It is far from a typical British period piece, and it’s definitely worth seeing on a large screen for the cinematography that blends the simple beauty of rural England with the devastation of war—bombs, gunfire, and bloodshed—in a deeply moving way.

Source on TMDB