An Army investigator infiltrates a Tokyo crime syndicate to solve a colleague's murder.

In 1954, a military train guarded by American soldiers and Japanese police is attacked as it travels between Kyoto and Tokyo. During the raid, which is carried out with great precision, an American sergeant is killed, and the train's cargo of guns and ammunition is stolen. The crime is investigated by Capt. Hanson, an American, and Japanese police inspector Kita, who, five weeks later, are concerned when a thief named Webber is shot with some of the stolen bullets. As Webber lies dying in a Tokyo hospital, he is questioned by Hanson and Kita, and although Webber was left for dead by his gang during a thwarted robbery, he refuses to implicate his cohorts, who presumably are responsible for the earlier crime. Webber, who is also an American, does reveal, however, that he is secretly married to a Japanese woman named Mariko. Among Webber's possessions is a letter from an American named Eddie Spanier, who wants to join Webber in Japan after his release from a U.S. prison. Three weeks later, Eddie arrives in Tokyo and finds Mariko, who is initially afraid that he is one of the men responsible for her husband's death. Eddie gains Mariko's trust with a photograph of himself and Webber, then warns her to keep quiet about her marriage so that she will not be in danger from Webber's killers. Later, Eddie goes to a pachinko parlor, in which patrons gamble on intricate machines similar to pinball machines. There, Eddie attempts to sell "protection" to the manager, but when he returns to discuss the matter again, he is beaten and warned to leave by racketeer Sandy Dawson and his henchmen, Griff, Charlie, Willy and Phil. Intrigued by Eddie's presence in Japan, Sandy arranges for him to be arrested, and Sandy's secret informer, who is connected to the police department, obtains Eddie's rap sheet. Convinced of Eddie's aptitude for crime, Sandy invites him to join his gang, which consists of former American servicmen who have been dishonorably discharged. After his acceptance into the gang, Eddie secretly meets with Kita and Hanson, for whom he is working undercover. Needing help from someone he can trust, Eddie asks Mariko to live with him as his "kimono girl," although he does not reveal his identity as a military police investigator. Hoping to discover who killed her husband, Mariko resides with Eddie despite being ostracized by her neighbors, who do not know that her relationship with the foreigner is platonic. As time passes, Sandy grows to trust Eddie, although Eddie is shocked during a robbery when a wounded gang member is killed by Griff to prevent him from talking. Eddie is also wounded, but Sandy makes an exception to his rule of killing fallen men and saves him. Eddie finally informs the worried Mariko that his real name is Sgt. Kenner, and that he is investigating Sandy. Meanwhile, Griff, Sandy's "ichiban" or "number one boy," becomes jealous of Sandy's reliance upon Eddie, and Sandy relieves the hot-headed Griff of his duties. The next day, Mariko, who has fallen in love with Eddie, notifies Kita and Hanson about a planned robbery, but Sandy's informant, reporter Ceram, warns him that the police are poised to capture him. After the robbery is aborted, Sandy kills Griff, whom he mistakenly assumes tipped off the police. Ceram informs Sandy of his mistake, and Sandy retaliates by setting Eddie up to be killed by the Japanese police during a robbery of a pearl broker. When the plan fails, Sandy is chased by the police up to a rooftop amusement park, but after an intense gunfight, Eddie succeeds in shooting and killing Sandy. Later, wearing his military uniform, Eddie walks with Mariko in a Tokyo park.