Charles Thénier returns from the beach to learn that his young son Michel was killed in a hit-and-run with a sports car. Grief drives him to keep a meticulous journal in which he states his plan to kill the offender, a vow that fuels the growing tension of the story and colors every encounter he pursues in the days that follow.
The investigation by the police stalls, leaving Charles with a dangerous determination and a shadow of doubt about who could be responsible. He begins to suspect the culprit might own some kind of workshop or garage, since there is no official record of a car going in for repairs. By chance, his hunch leads him to a startling connection: the actress Hélène Lanson was the passenger in a damaged car on the day of the accident. Using his pen-name, he seduces her and learns that the driver was her brother-in-law, Paul Decourt. Hélène admits she has suffered from depressive anxiety, and Charles presses for more details, ultimately convincing her to reveal more. She agrees to accompany him on a trip to Brittany to visit her sister’s family, hoping to escape some of the traumatic memories that haunt her.
On the road, Charles observes Paul’s true nature: he is detestable and cruel to his wife, and he is deeply disliked by his teenage son, Philippe Decourt. He also learns that Paul is a serial womanizer and that Hélène once slept with him. Although Charles wrestles with whether to kill Paul, he nonetheless saves Paul from a dangerous cliff-fall, sensing there is more at stake than revenge. Philippe confides in Charles about his own longing to kill his father, adding a complicating layer to the family dynamics and Charles’s growing sense of moral ambiguity.
Driven by conflicting impulses, Charles decides to eliminate Paul in a staged sailing accident and buys a boat for that purpose. But the plan is complicated when Paul pulls a gun during their voyage and reveals that he has read Charles’s journal and even entrusted it to his solicitor to hand to the police should anything happen to him. When they return to the harbor, Paul ejects Charles from his house, and the would-be killer appears to abandon the scheme, driving away with Hélène. A television report announces Paul’s death by poisoning and pleads for Charles and Hélène to return, which they do.
The police confront Charles with theories that the diary’s release was staged to deflect suspicion, and Charles is briefly arrested. In truth, Philippe soon confesses to the murder and provides the key piece of evidence—the poisoned medication bottle—as proof, clearing Charles in the eyes of the authorities because Charles had already left before Paul’s death occurred. Yet the web of deception continues to unfold as the couple returns to their hotel; Charles, weary and unsettled, promises to recount the entire tale to Hélène the next day.
In a final, haunting coda, Charles leaves a note for Hélène explaining that Philippe’s confession was an attempt to shield him, and that Charles himself is the actual murderer. He requests that she share his confession with the police, and he resolves to punish himself and vanish from view. As the film closes, Charles sails away across the ocean, while a Brahms line lingers in voice-over: > For that which befalls man befalls the beast, as the one dies, so dies the other.
