Picaresque Irish adventurer Paddy Blackthorn is caught attempting to steal an interstellar space drive and is sentenced to death by the ruling council of mutant humans. The mutants' creator bequeathed them the secret of the drive, and with it a monopoly on space travel, which allows them to dominate normal humans.

During his escape, Paddy discovers that the knowledge of how to manufacture the engines has been stored in five gold rings, one for each mutant race. The rings are hidden in five secret locations for safekeeping. With the help of a beautiful human secret agent, Fay Bursill, Paddy searches the home planets of each of the mutant species, in the hope that Earthfolk will be able to resume their rightful place in space.