In 20XX, mysterious colossal robots called Giant Robo (GR) begin appearing worldwide, destroying cities and spreading fear. Their next target is Japan. The story follows Daisaku Kusama, a diving‑shop employee who encounters a mysterious girl named V. She leads him to the ruins on Minami Yonaguni Island, where he signs a contract with the enormous robot Giant Robo 1 (GR1). After forming an alliance with UNISOM, the leading organization, Daisaku is authorized to command GR1. He then battles the antagonist group GRO, creating intense drama. As the 21st‑century Giant Robo confronts its foes, it forces the world to question the true meaning of its power.