The Captain America Republic Serials were loosely based on the Timely Comics era Captain America.
They focused on Captain America, secretly District Attorney Grant Gardner, trying to thwart the plans of The Scarab, museum curator Dr. Cyrus Maldor, to steal the *ahem* "Dynamic Vibrator" and "Electronic Firebolt". Devices that could be used as super-weapons of mass sexual innuendo or something like that.
The serial ran in 15 chapters and was the most expensive serial Republic ever made. It would become the first Marvel Film ever and the last superhero serial of the Golden Age.
[Above, Promotinal shot of Dick Purcell as the title charecter c. 1945]