Sometime in the distant future, the planet Metropolis (once known as Earth) is a peaceful utopia led by a leader called The Great Brother. His world is threatened, however, by his brother Graal who has escaped from a prison satellite, stolen a warship and enlisted the help of a mad scientist named Dr. Kraspin. Kraspin has a plan to make for Graal an army of unstoppable supersoldiers he calls "Humanoids," but to mutate them he requires the powerful element Kapitron that is being kept at the Grovan Institute on Metropolis.