In the climax of the film, Briony Tallis (played by Saoirse Ronan as a child and Romola Garai as an adult) comes to terms with the consequences of her actions and seeks redemption for the false accusation she made against Robbie Turner (played by James McAvoy), which ultimately tore apart his life and his relationship with Cecilia Tallis (played by Keira Knightley).
Throughout the movie, Briony grapples with guilt and remorse after falsely accusing Robbie of raping her cousin, Lola Quincey (played by Juno Temple). As she grows older, she becomes a successful novelist but remains haunted by the events of that fateful day and the impact of her actions on the lives of those around her.
In the final moments of the film, it is revealed that the entire story was written by an elderly Briony, who admits that she fabricated the events of the narrative as a way to atone for her past mistakes and seek forgiveness from Robbie and Cecilia, both of whom died during World War II.
The film ends with Briony reflecting on the power of fiction to rewrite history and reshape reality, acknowledging that she can never truly undo the harm she caused but hoping that her words can offer some form of redemption and closure for those she wronged.
Through its poignant and thought-provoking conclusion, “Atonement” explores themes of guilt, forgiveness, and the consequences of our actions, leaving viewers with a profound sense of reflection and introspection.