In the era of 20XX, the world is under siege by colossal robots known as Giant Robo (GR), leaving a trail of destruction in their wake. Amidst this chaos, Japan becomes the imminent target of these mechanical behemoths. Daisaku Kusama, a humble employee at a diving shop, crosses paths with a enigmatic girl named V, who guides him to forge a pact with the mighty Giant Robo 1 (GR1) at the ancient ruins of Minami Yonaguni Island in Okinawa. Embracing his newfound role as GR1's commander under the auspices of UNISOM, the primary organization, Daisaku embarks on a harrowing journey against the malevolent GRO faction, unraveling a tapestry of intense conflicts. As the battles escalate, the 21st-century Giant Robo challenges the world to contemplate the profound significance of its formidable power.