The story begins with a tragedy when British newlyweds, Mark and Laura Needham, are vacationing on a hill overlooking the French village of Volnay. While Mark is asleep, Laura goes for a short stroll, only to be brutally assaulted and strangled by an unknown attacker.

Frustrated by the slow progress of the local police—led by Commissaire Antoine Cirret of the Sûreté Nationale—Mark takes matters into his own hands. Cirret secretly enlists Mark's help to canvas a specific Parisian neighborhood. Through this search, Mark encounters Theo Bondet, a local grocer who is secretly a serial killer responsible for Laura's murder and several other cold cases.

Mark manages to closely befriend Theo to gather evidence, but his growing psychological obsession with revenge completely consumes him. He plans to murder Theo on a boat, but the confrontation backfires spectacularly, and Theo kills Mark instead, dumping his body in a river.

The perspective then shifts to Mark’s friend, a musician named Alec Trevor, and a woman named Andrée. Discovering Mark's death, Alec devises a complex, vigilante trap. Andrée disguises herself as a prostitute to lure Theo into an aggressive trap, while Alec uses psychological warfare to force a confession out of the killer right before the police finally close in.