On the trail of a spy ring intent on stealing government radar plans, world-famous Chinese-American detective Charlie Chan, now working as a federal agent, enlists the aid of Captain Flynn of the New York City police. Unfortunately, Flynn follows Chan's one lead to the spies, a scientist named Rausch, too closely, and Rausch is murdered by his mysterious, unnamed boss. Discovering that the murderer escaped in the stolen car of Diane Hall, a radio performer, Chan's and his two assistants--his son Tommy and his chauffeur, Birmingham Brown--go to the Cosmo Radio Center, where they find a footprint identical to one left at the murder scene. Meanwhile, Ralph Brett, the studio manager, telephones the spy ring leader, who uses the Western Union telegram service to warn Brett to be more careful or meet the same fate as Rausch. Later, Chan goes to the Hamilton Laboratory, which is located in the same building as the radio center, and is told of numerous, unsuccessful attempts to break in and steal the radar plans from the laboratory's safe. Chan then places phony radar plans in the safe in case the spies are more successful in the future. After actress Gloria Bayne attempts to blackmail Brett, having found his matches in Diane's recovered car, she is killed by an unidentified poison. Realizing that Chan is onto him, Brett asks his boss for help in escaping, but the spy instead kills him using a trap door in the service elevator. Upon finding the murdered studio manager, Chan has a voice impersonator call the spy leader, who, thinking that Brett is still alive, orders him to use the service elevator once more. After escaping the trap, Chan questions the people who worked with Brett and Gloria. Later, Willie Rand, an actor, is killed while taping a television show after telling Chan that he may have uncovered some information. The detective soon discovers that both Gloria and Willie were killed by a poisonous gas which was activated by cigarette smoke. After searching the building and finding the spy leader's office, the leader is chased through the radio studio by Chan, Tommy, Birmingham and the police, only to be killed by the elevator's trap door. In the basement of the building, they discover the dead body of Mrs. Marsh, the ruthless radio sponsor, and Chan declares the case solved.