Hit man Jef Costello (Alain Delon) goes through an elaborate set of rituals before carrying out a hit on a nightclub owner. Always thorough and cool in his job, Costello is disconcerted to discover a witness to the killing, the club's female piano player. But before he can act, the police arrest him in a sweep of suspects. Released when the main witness does not come forward, Costello goes from being the hunter to the hunted, trailed by the determined police as well as his crime bosses.