Il y a des millions d'années, pour fuir les radiations Getter émises par une étoile lointaine, les dinosaures abandonnèrent la surface de la Terre et bâtirent un empire souterrain en attendant l'occasion de reprendre possession de la planète. Alors qu'ils estiment leur temps venu, le professeur Saotome découvre leurs plans et, à l'aide de la technologie Getter, construit une machine de guerre sans égale aucune : Getter Robo. Mais ce mecha fabuleux exige des pilotes à la hauteur de sa puissance...

In the year 200X, Doctor Rando, a genius scientist gone mad, has broken the peace on the Earth by trying to conquer the world from his Polar base, using an army of his Metal Beasts, super robots developed by him to conquer the world and led by his genetically created officers. No regular weapons seem to work against the Metal Beasts. To combat him, Japan's Defense Agency request the cooperation of NISAR, the Japan International Aerospace Corporation, which has developed a robot, the Getter Robo. Dr. Tachibana, the head of the Getter Robo project, is reluctant to use the robot for battle. Despite this, Getter Robo, piloted by Go Ichimonji (later joined by Sho Tachibana and Daido Gai), is able to defeat a Metal Beast with the sacrifice of Shinichi, the son of Dr. Tachibana. This prompts the improvement of Getter to make it a transforming machine piloted by three persons, capable of defeating the Metal Beasts and stopping Dr. Rando from conquering the world.