When a group of children discover a strange cave at the beach, their lives are forever changed. Inside they find a hide out filled with computers and a man named Kokopelli who gives them a curious offer: to participate in a special game in which they save Earth from fifteen giant monsters. To defeat the invaders, he will give them a powerful mecha of black armor. The children eagerly sign the contract, name their new weapon Zearth, and must now take turns to pilot it; but the 'game' is in fact all too real and the consequences of battle become the stuff of nightmares. With no option to cancel the contract, is there any way to stop the game before it is too late for all of them?