Year 2041. A new enemy called Zerabaia suddenly appears in the solar system, destroying all military installations, found him on the road. But,mighty Klein Sandman is already aware of this planned invasion and secretly prepared his army for battle. His trump card - Gravion, giant robot, the embodiment of the latest technology of the human mind. Meanwhile, a young man named Eiji, in search of the lost sister secretly enters Sandman organization Klein.It was there that he meets another young man by the name of Toga... These two heroes, along with four other people now will fight Zerabaia on board the robot Gravion, to protect the earth from impending threats.