In the cool, rain-soaked streets of Paris, a solitary hitman known for his cold precision, Jef Costello, lives governed by a strict personal code akin to that of the Japanese samurai. To maintain his untraceable anonymity, Jef exists in a world marked by isolation and meticulous attention to detail. His only connections seem to be with his pet bird and his girlfriend Jane, who provides a clear alibi for him. Jef is hired for a well-paying job at a nightclub — his task is to execute the owner. With his trademark methodical efficiency, Jef carries out the hit, skillfully avoiding much notice, but his exit does not go unseen. A jazz pianist at the club catches a fleeting glance of him.

The police, led by the diligent and intuitive Inspector Mattei, quickly zero in on Jef as their prime suspect. They orchestrate an extensive lineup, where the witnesses from the nightclub are gathered in hopes of identifying the killer. Jef's alibi holds up under questioning, thwarting the police's attempts to directly link him to the crime. Nevertheless, Mattei is not convinced of Jef's innocence and launches an elaborate cat-and-mouse game to track his every movement. The inspector's relentless surveillance begins to close in, threatening Jef's iron-clad façade of invincibility. With the authorities on his tail and his employers doubting his reliability, Jef is drawn into a tightening web of complications that begin to strip away at his carefully constructed existence.