The Master Key is a 1945 Universal movie serial.

Plot
Nazi spies, led by the mysterious “Master Key”, kidnap Professor Henderson in order to acquire his “Orotron machine” which is capable of extracting gold from sea water. FBI agent Tom Brant, aided by reporter Janet Lowe and Detective Lt. Jack Ryan, attempt to rescue him and crack the Nazi spy ring.

Cast
Milburn Stone as FBI Agent Tom Brant
Jan Wiley as Janet Lowe, a reporter
Dennis Moore as Detective Lt. Jack Ryan
Addison Richards as Gerhard Doenitz, alias Garret Donahue, private investigator and Nazi agent
Byron Foulger as Professor Elwood Henderson
Maris Wrixon as Dorothy Newton
Sarah Padden as Aggie
George Lynn as Herman, the “Spearpoint Heavy” (chief henchman)
Russell Hicks as Police Chief Michael J. O’Brien
Roland Varno as Arnold “Hoff” Hoffman, alias M-3
Lash LaRue as Migsy, a street urchin. This was La Rue’s screen debut.[1]
Jerry Shane as Dan, boys’ club member
Neyle Morrow as Spike, boys’ club member
John Eldredge as Walter Stark, alias M-6