Fifteen middle schoolers are enjoying a seaside summer camp until one child guides the group into a cave that will permanently change their lives. Inside, they find several computers and a man named Kokopelli, who proposes a game: piloting a massive robot to repel enemies attacking Earth. The students agree to sign a contract by touching a shield-shaped object, and Kokopelli transports them to the cockpit of Zearth, the mecha they are to pilot. When an enemy mecha emerges from the sea, Kokopelli demonstrates how to mentally control Zearth and destroys it. The children are thrilled by the idea of defending Earth, but the situation is not what it appears. Zearth is actually powered by life energy, and the pilots die at the end of every battle. Having signed away their lives, the children must now use their remaining time on Earth to defend it.