The year is 20XX and the looming threat of enormous robots destroying cities has the world on edge. The next target for the destructive force known as "Giant Robo (GR)" is Japan, plunging the entire planet into fear. Daisaku Kusama, an employee at a diving shop, is visited by the enigmatic V who takes him to the ruins of Minami Yonaguni Island in Okinawa to forge a contract with the powerful "Giant Robo 1 (GR1)". After UNISOM, the main organization, approves Daisaku as the commander of GR1, he engages in high-stakes combat against GRO, the enemy organization, stirring up drama. As the 21st century rolls on, the emergence of Giant Robo begs the question of its true purpose and power.