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As a wave of sabotage threatens the security of the country, Saxby, the head of the U.S.  Secret Service, assigns agent Brass Bancroft to impersonate the deceased spy Steve Swenko.  Gabby Watters, Brass's assistant, finds a letter in the dead spy's shoe, addressed to Joe Garvey, the leader of a patriotic society that is being investigated by the Rice Committee on Un-American Activities.  Consequently, Brass, posing as Swenko, sets out to contact Garvey.

Brass's task is complicated when Hilda Riker, Swenko's wife, appears at his hotel looking for her husband.  Brass tells Hilda that Steve is dead and he is carrying out his mission, but Hilda recognizes him as a federal agent and alerts Otto Brennerman, one of Garvey's operatives, to set a trap for Brass at her apartment.  Luckily, Gabby, posing as a taxi driver, follows Brass to the apartment and comes to his rescue, arresting Hilda and Otto before they can notify Garvey about the deception.

An unwitting Garvey then assigns Brass to board a dirigible on which an inertia projector, a defense weapon capable of paralyzing electical currents at their source, has been installed.  Once aboard Brass is to contact Rumford, a spy posing as secretary to Dr. Finchley, a member of the League of Nations.  Rumford orders Brass to destroy the dirigible while he steals the plans for the inertia projector, but before Brass can act, Otto escapes and warns Garvey and Rumford that Brass is really a government agent.

Soon afterward, the dirigible crashes during a storm, and Rumford steals the plans and leaves the unconscious Brass to perish in the crash.  After Brass is rescued and taken to a Navy hospital, Rumford discovers that he is still alive and warns Garvey, who prepares to fly Rumford and the stolen plans across the border.  Before they can escape, Brass regains consciousness and tells Saxby about Rumford.  Saxby then alerts the airports to watch for Garvey's plane, and in a spectacular air chase, Garvey's plane is shot down by the inertia projector, sending both spies to their death in a burst of flames.