Universal's 64th of 69 sound-era serials (between "Jungle Queen" with Ruth Roman and their 2nd version of "Secret Agent X-9" with Lloyd Bridges), is set in 1938 with Nazi agents kidnapping scientist Professor Ellwood Henderson in order to get his Orotron machine that can extract gold from seawater. Operating under the secret leadership of "The Master Key", with the operatives headed by a German agent posing as a private investigator named Garrett Donovan, the gang is trailed by Federal agent Tom Brant, aided by police detective Lieutenant Jack Ryan and newspaper reporter Janet Lowe. Also aiding in the quest is Police Chief William J. O'Brien and his secretary Dorothy Arnold. It takes Brant nearly 12 of the 13 episodes to figure out that since the Nazis learn of his and Ryan's plans almost before they clear police headquarters, then "The Master Key" must be somebody close to them.