In the year 20XX, colossal mysterious mecha known as "Giant Robo (GR)" begin appearing across the globe, laying waste to cities. Fear grips the Earth, and Japan is their next target. The protagonist, Daisaku Kusama, a diving shop employee, meets an enigmatic girl named V, who guides him to the ancient ruins on Minami Yonaguni Island in Okinawa. There, he forms a contract with an enormous robot called "Giant Robo 1 (GR1)."
Daisaku then enters into a contract with UNISOM, the main organization, and is granted approval to command GR1. His battle against the antagonist group GRO commences, leading to numerous dramatic events. Eventually, the Giant Robo of the 21st century will pose a question to the world regarding the true nature of its power.