The Lost Planet

This has the distinction of being the last Hollywood serial to be released in theatres. Essentially, it's a tale of two metals -cosmonium (which the ruler of Planet Ergro, Michael Fox, needs to rule the universe) and dornite (which renders journalist Judd Holdren invisible in times of crisis). As for the rest, it's a standard story about a kidnapped professor, his plucky daughter and various solar-thermo lasers, which only serves to confirm that even the most clued-in villain could never get decent henchmen, as an escaping Dr Grood's cosmojet robot sets his course for infinity instead of deep space.