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.

An internationally-notorious criminal scientist returns to the US to sell his latest invention, a disintegrating gas, to a foreign power. When he arrives, however, he is spotted by a young Coast Guard man, whom he kills - and thus earns the enmity of the entire US Coast Guard, but especially the murdered Guardsman's older brother who, together with his reporter-girlfriend and her comical photographer, vigorously sets to tracking him down and interfering with his plans to develop the city-melting gas in quantity. A Republic Serial in 12 Chapters.