One summer, fifteen kids innocently wander into a nearby seaside cave. There they meet a strange man who invites them to play an exciting new video game. This game, he explains, pits one lone giant robot against a horde of alien invaders. To play the game, all they have to do is sign a simple contract. The game stops being fun when the kids find out the true purpose of their pact.

Two children remain to fight for Earth's survival, and one of them isn't from this Earth at all. Zearth's next pilot, Yoko Machi, has been possessed by the mind of a refugee from a parallel world. Before she goes into battle, she and her fellow pilot Jun Ushiro face a struggle of a different kind, as they confront the many victims left behind in Zearth's wake.