The Master Key (1945)

Starring Milburn Stone, Jan Wiley, Dennis Moore, Addison Richards, Byron Foulger
Directed by Ray Taylor & Lewis D. Collins
(actor & director credits courtesy IMDB.com)

A government agent and a police lieutenant try to defeat the plans of a ring of Nazi agents in America, who have acquired a machine that extracts gold from sea water.

I wouldn't call this a great movie serial, but it is a fun one, with Universal stalwart Stone and serial veteran Moore trying to track down the secret location of the Nazi agents with the aid of reporter Jan Wiley.  Richards is quite good as the chief Nazi agent, who along with his cohorts all have keys that allow them to communicate with "The Master Key," a mysterious voice that gives them their orders.  The serial could use some more fantastic elements in my opinion, but there's some interesting plot devices in the screenplay, with reporter Wiley employing a group of street kids to do her legwork, a secret relationship between Wiley and one of the Nazi agents, and some twists and turns that always keep the villains a step ahead of our heroes until the conclusion.  Universal horror fans will recognize a number of the music cues, including the title theme which repurposes the main title of House Of Frankenstein.