The year is 20XX. Mysterious massive robots known as "Giant Robo (GR)" began appearing worldwide, destroying cities. The world was gripped by fear. Their next target is Japan! The protagonist of this series, Daisaku Kusama, who works at a diving shop, meets a mysterious girl simply called V. She leads him to form a contract with the colossal robot "Giant Robo 1 (GR1)" at ancient ruins on Minami Yonaguni Island in Okinawa. He signs a contract with UNISOM (the good guys) and is approved as the commander of GR1. Thus, his battle against GRO (the bad guys) begins, creating numerous dramatic situations. Soon, the Giant Robo of the 21st century will ask the world about the true meaning of its power.