Warning: full spoilers, including the ending.
The film moves in two halves hinged on a single gunshot. In the first, Meursault buries his mother without crying, returns to Algiers, sleeps with Marie, and is drawn into the orbit of his neighbour Raymond Sintès, a pimp who has beaten his Arab mistress and wants Meursault's help writing a letter to lure her back. Meursault agrees — not out of malice, but because refusing would take more conviction than he possesses. The woman's brother and his friends begin shadowing Raymond.
The confrontation follows them to a beach outside the city. After an initial scuffle, Meursault takes Raymond's revolver so that Raymond will not use it, then walks back alone into the glare. Meeting the Arab man by a spring, he fires — once, then four more times into the motionless body. Camus's crucial detail survives: there is no motive. Heat, light, sweat in the eyes, a knife-blade's flash. The four extra shots, the thing no court can forgive, are what the trial will treat as proof of a monster.
The trial. The prosecution barely litigates the killing. It litigates the funeral. Witnesses describe a son who did not want to see his mother's body, who smoked and drank coffee at the vigil, who went swimming and to a comedy the next day. Marie's testimony about their happiness is turned against her. Raymond's disreputability becomes Meursault's. Asked why he killed, Meursault says it was because of the sun — the courtroom laughs — and the prosecutor concludes that a man who buried his mother with the heart of a criminal has no place among men. He is sentenced to death by guillotine.
The cell. Refused appeal-shaped hope, Meursault rejects the chaplain's offer of God, first calmly, then in a torrent of rage: nothing matters, no life is worth more than another, and everyone is privileged in the same way, condemned in the same way. Emptied by the outburst, he opens himself for the first time to what he calls the tender indifference of the world — a universe that resembles him, that promises nothing and therefore betrays nothing. He finds himself happy, and wishes only for a large crowd of spectators at his execution, greeting him with cries of hate. It is the book's most quoted turn, and the film keeps it: the coldest man in French literature arriving at something like peace.
Ozon's addition. Then the film goes where the novel cannot. A coda gives the murdered man — nameless in Camus, an abstraction in the courtroom — a face, an inner life, and a widow who visits his grave. The retroactive meaning is severe: everything we have just watched, including Meursault's hard-won lucidity, has been a story told from inside the colonial frame. The trial's real absurdity was never that it judged a man for not weeping; it was that it could spend days on his dry eyes and never once ask who was killed. Placed after Meursault's exaltation, the coda withholds the catharsis the ending would otherwise deliver, and reframes the prologue's newsreels as the film's true opening argument rather than mere period scene-setting.