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.

Zearth's surviving pilots make a promise to each other: they'll never break down, they'll never cry, and no one's to blame. Yet after Maki's battle breaks open the core of her opponent a frightening discovery may test their new resolve. Inside, the children come upon a terrible shock--the truth about who they're being driven to destroy.