In the year 2041 AD, a new enemy — the Zeravire — suddenly appears in the solar system, destroying all military outposts on its path towards Earth. However, a wealthy man named Klein Sandman is already aware of this planned invasion and has been secretly preparing his army for battle. His trump card is Gravion, a giant robot that utilises gravity as its source of energy.

Meanwhile a young man named Eiji secretly enters Sandman’s base, searching for his missing sister. It is there that he meets another young man by the name of Toga. These two must now fight together with four other individuals aboard the super robot, Gravion, to fend off the Zeravire threat.