This is a science fiction story depicting a battle between the "rocket-man" Ambassador Magma created by "Earth" (the creator of the earth), and evil spaceman Goa.
Newspaper writer Murakami Atsushi and his family wake up one morning to find they have travelled 200 million years back in time. Looking out their window they see dinosaurs were walking around outside their house. This is, in fact, the work of a spaceman named Goa, who sent them back into the past in a show of his power. Proclaiming that he would take over the earth, Goa dares Murakami to report his plans in the paper. Coming back to the present, Mamoru, son of Murakami, is taken by a giant named Magma to the underground of a volcano island, where he meets the creator of the earth (called "Earth"). To squash Goa's ambitions, 'Earth' creates three "rocket-men" named Magma, Mol and Gum.
Magma gives Mamoru a whistle he can use to call him and the other rocket-men whenever he is in trouble, and they fight together against Goa. But Goa has already started his earth invasion project, sending creatures that can turn themselves into humanoids, called "Hitomodoki."