On 1 November 1941, American-born Japanese businessman Ito Takimura meets with Japanese spies in Tokyo. Takimura is welcomed into the Black Dragon Society and urged to use his importing business as a front for gaining information about Pacific Coast industries and defense plans. When Takimura returns to his home in the Little Tokyo section of Los Angeles, he organizes his compatriots, Kingoro, Satsuma and German-American spy Marsten. As the weeks pass, police detective Michael Steele becomes suspicious that there are spies in the area, but Takimura assures him that he is mistaken. Mike's suspicions are heightened, however, when he stops two boys from fighting and one of them, Satsuma's son Suma, brags that his father talks to Tokyo every night on a radio. Mike asks an old friend, Oshima, to investigate Satsuma, then goes to the radio station, where his girl friend, Maris Hanover, works as a commentator. Maris derides Mike's fears of espionage, but later that night, after Oshima fails to meet them, she accompanies Mike to Oshima's apartment, where they discover that the Okono family has moved in and claim no knowledge of Oshima's whereabouts. Mike rushes to Satsuma's house to search for the radio, but finds no transmitting devices or evidence concerning Oshima's disappearance. Satsuma introduces Mike to Teru, a beautiful young woman Satsuma claims is his daughter, but who is really the mistress of Hendricks, Maris' boss. The German-American Hendricks is secretly in league with Takimura and the others, and allows them to use the radio station's transmitter to relay signals to Japanese ships late at night. After leaving Satsuma's home, Mike goes to the morgue, where his pal Jerry shows him a decapitated corpse. Mike recognizes the body as Oshima's from a scar on his shoulder and realizes that the mode of killing indicates that his friend was murdered by the Black Dragon Society. On 4 December 1941, police captain Wade informs Mike that pressure from prominent Japanese businessmen has resulted in his transfer to another precinct, to take effect in four days. Mike asserts that the transfer proves he is close to catching the spies, and his refusal to cease his investigation prompts Takimura to use Teru as bait in a trap. On the night of 6 December, Teru invites Mike to Satsuma's house, where she drugs him. As Mike sleeps, Hendricks and Takimura kill Teru and make it look as if Mike murdered her while trying to assault her. Mike is arrested for the murder, and the next morning, is in prison when he learns of the Japanese attack on Pearl Harbor. Mike then escapes from jail and soon discovers where Takimura, Hendricks and the others meet. With Maris' help, Mike tricks the spies into revealing their activities while the police listen, and soon the gang is rounded up. After Japanese Americans on the West Coast are taken to internment camps, Little Tokyo becomes a ghost town, and Maris comments on her radio show that loyal Japanese Americans must suffer along with the disloyal in the interest of national security. She then reads an excerpt of Robert Nathan's poem "Watch America," and urges Americans to maintain their vigilance against espionage.