This tale began on a snowy night eight years ago… On the snowy plains of Hokkaido, the Amamis are given a baby by a mysterious mecha-lion. This baby holds the key to the fate of mankind. In 2005, a giant robot appears suddenly in Tokyo Bay and begins attacking Shinjuku. Among the children caught up in this attack is Mamoru Amami, the boy who was raised by the Amamis. The secret defense organization GGG's Captain of the Mobile Unit, Gai Shishioh, and the mecha-lion Galeon are dispatched to stop the giant robot which continues to attack central Shinjuku. Galeon, who had be dormant in a GGG storeroom, awakens as though responding to the danger facing Mamoru. Gai and Galeon fuse to form GaiGar, and face off against the robot named EI-02. GaiGar then fuses with the GaoMachines in order to become the mobile mechanoid GaoGaiGar, and defeats EI-02. GaoGaiGar removes the robot's core and is about to crush it, but is stopped by the boy Mamoru, who is surrounded by a dazzling light.