A mad scientist named Dr. Satan plots to steal key pieces of technology to enable him to build an army of robots based on his prototype to conquer America. The only one standing in his way is Bob Wayne, who fights Satan as the enigmatic Copperhead. Mysterious Doctor Satan is a 1940 film serial named after its chief villain. Doctor Satan's main opponent is the masked mystery man, "The Copperhead", whose secret identity is Bob Wayne, a man searching for justice and revenge on Satan for the death of his step-father. The serial charts the conflict between the two as Bob Wayne pursues Doctor Satan, while the latter completes his plans for world domination.

Although ostensibly the grand-son of the legendary hero, Clayton Moore's Ken Mason is little more than a cowboy in a black mask in this 12 chapter Republic serial. Mason, the head of the telegraph line work crew, assumes his ancestor's trade-mark mask (but not whip) in order to prevent a local czar (Roy Barcroft) from sabotaging the burgeoning telegraph line. Pamela Blake, a brunette starlet formerly known as Adele Pearce, played Mason's imperiled girlfriend, and the serial also benefitted from the usual competent work of Republic's great stunt-performers, including Dale van Sickel, Tom Steele, Eddie Parker, and Joe Yrigoyen.