After the final defeat of the Dinosaur Empire and the death of Musashi Tomoe in the original Getter Robo series, Dr. Saotome (Dr. Copernicus in Starvengers), creator of Getter Robo, fears that the peace the Getter Robo team has won will be short lived and that even greater enemies will appear. Dr. Saotome's fears are justified when the Hundred Demon Empire (Hyakki Teikoku, also known as the Pandemonium Empire in Starvengers) appears. Composed of horned, devil-like alien humanoids and possessing a technology advanced enough to build giant demonic robots, this militaristic organization intends to steal Saotome's Getter Generator to fulfill their own goals of world domination.

However, Dr. Saotome is prepared with the creation of an even more powerful Getter Robo, Getter Robo G and a new Getter Robo base. Also, with Musashi dead, Dr. Saotome needs a third pilot, which he finds in baseball player Benkei Kuruma