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John Keller, the inventor of a revolutionary new plane, is joined on his flight to the Radex Airplane Factory in California by detective Nick Carter, who masquerades as Robert Chalmers, the new assistant to plant president Hiram Streeter. Once they are airbound, the pilot feigns motor trouble and lands the plane to rendezvous with a group of spies intent upon stealing Keller's plans. Nick foils their plot, however, and, guided by the flying expertise of stewardess Lou Farnsby, the plane and its contents safely reach California. At the Radex Plant, Streeter explains to Nick that top secret blue prints have been stolen, despite tight security. The future of the plant hinges upon the development of Keller's new high speed plane, and consequently, Streeter has sent for Nick Carter to investigate. Nick's list of suspects includes Lou, and he asks Streeter to reassign her to the infirmary. Nick is aided in his investigation by Bartholomew, a bothersome amateur sleuth who appoints himself the detective's assistant. When sabotage destroys the test flight of the new plane, Nick begins to suspect Keller until the inventor is murdered. Noticing that a string of accidents at the plant have coincided with the disappearance of the blue prints, Nick asks Lou to watch Dr. Frankton at the infirmary. When Nick catches one of Frankton's patients with photos of the plans taped underneath his bandages, Frankton realizes that he is about to be arrested and arranges for Lou to be taken hostage. Demanding safe passage from the plant, Frankton makes his getaway by boat. Nick and Streeter then trail him by plane and alert the harbor patrol of his location. After their plane is downed during a daring shootout at sea, Bartholomew and the harbor patrol arrive to rescue Nick and Streeter and arrest the spies.