This Man Must Die

Charles Thénier returns from the beach to learn his young son Michel has been killed in a hit-and-run. The tragedy drives him to vow revenge and he begins keeping a journal in which he declares his intention to kill the offender once he finds him.

The police investigation yields no leads and stalls. Charles starts following a hunch that the driver was connected to a garage because there is no record of a car going in for repairs that day.

While pursuing his hunch, Charles learns that Hélène Lanson was the passenger in the car on the day of Michel's death. Under his pen-name Marc Andrieux, he seduces her and learns that the driver was her brother-in-law Paul Decourt. Hélène confesses she has suffered depressive anxiety.

In Brittany, Charles confirms that Paul is detestable, cruel to his wife, and hated by his teenage son Philippe. He also learns that Hélène once slept with Paul. Philippe expresses to Charles his own desire to kill his father.

Charles wrestles with whether to kill Paul. He decides to kill him in a staged sailing accident and buys a boat for the plan. During the preparations, he rescues Paul from a cliff-fall, showing his reluctance to go through with murder.

At sea, Paul pulls a gun on Charles and reveals that he has read Charles's diary and has given it to his solicitor to deliver to the police if anything happens to him. Charles returns to the harbour.

Back in harbour, Paul throws Charles out of his house and Charles abandons his plan to murder Paul, driving away with Hélène.

A roadside restaurant broadcasts Paul’s death by poisoning and pleads for Charles and Hélène to return. The police speculate that Charles engineered the diary’s discovery to deflect suspicion.

Charles is arrested, but the police cannot find the poisoned bottle that would prove Paul’s death by poisoning. He had left before the poisoning.

Philippe confesses to the murder and provides the poisoned bottle as proof, clearing Charles as the killer and prompting his release.

Back at the hotel, Charles is weary and leaves a note explaining that Philippe’s confession saves him and that Charles is the real murderer. He urges Hélène to share the confession with the police and vows to punish himself and vanish.

The film ends with Charles sailing into the ocean, reciting a line from Brahms and reflecting on fate: the consequences befall both the hunter and the hunted.