Mysterious Doctor Satan (1940) is a Republic Studio 15 chapter film serial named after its chief villain. Doctor Satan's (Edward Ciannelli) main opponent is masked mystery man The Copperhead (Robert Wilcox).
The serial charts the conflict between the two as Bob Wayne (Robert Wilcox) searches for justice and revenge while Doctor Satan completes his plans for world domination. Governor Bronson, who raised Wayne from childhood after the death of his parents, is killed at the hands of a world-domination-seeking mad scientist called Doctor Satan. Fearing his death might be at hand, as it has been for everyone else who had opposed the Doctor, the Governor first confides in Wayne with a secret about his past. Bob's father was really an outlaw in the Old West, who fought injustice while wearing a chain mail cowl and leaving small coiled copper snakes as his calling card. Following his guardian's death, Wayne decides to adopt his father's Copperhead persona and cowl. Doctor Satan, meanwhile, requires only a remote control device invented by Professor Scott to complete his army of killer robots and gain all the power and riches he desires. The Copperhead fights Doctor Satan, rescuing the Professor and others and preventing the Doctor from completing his plot.
Mysterious Doctor Satan was originally planned as a Superman serial for Republic, but the license National Comics provided to the Fleischer to make their Superman cartoon series was exclusive and therefore prevented other film companies from using the character at the time, even in a non-animated production. The script was subsequently reworked with a new character standing in for Superman. The Copperhead's love interest, Lois, had only her surname changed between these drafts, while his secret identity mimicked Batman's.
Harmon and Glut describe Mysterious Doctor Satan as "one of Republic's best serials... [which] set the pace for others that followed." They go on to narrow it down to one of the five or six greatest serials Republic ever made. Many people involved in the serial are singled out for praise but the main one is Ciannelli as Doctor Satan, a character who steals the show from the relatively bland Copperhead.