The Master Key is set in 1938. 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

text from wikipedia