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.

Having no interest in boys her own age, young Chizu develops a crush on her schoolteacher. After a few tentative secret dates, her relationship with the older man takes an unexpected and violent twist. When Chizu finally takes control of Zearth, she's got two things on her mind: save the planet and exact her revenge on her ex-lover.