The year is 2041 A.D. Zeravire, a new adversary, arises out of nowhere in the solar system, destroying any military structures it comes encounter. However, a rich man named Klein Sandman is already aware of the impending invasion and has covertly been preparing his army for fight. Gravion, a massive robot that uses gravity as a source of energy, is his trump card. Meanwhile, a young guy called Eiji joins Sandman's headquarters undercover in quest of his lost sister. It is there that he meets another young guy named Toga... To stave off the Zeravire danger, these two must now battle with four other people onboard the powerful robot Gravion.