The film takes place during a heatwave in the middle of summer in post-war Tokyo. Rookie homicide detective Murakami (Toshiro Mifune) has his Colt's Manufacturing Company pistol stolen in a crowded trolley ride. He chases the pickpocket, but loses him. Remorsefully, he reports the theft at headquarters. After some preliminary investigation, he then goes undercover in the city backstreets for days, trying to infiltrate the illicit arms market. He eventually picks up the trail of a gun racket.