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.

Saving the world is hard.
Saving yourself is even harder.

Giant robots are invading Earth and fifteen teenagers must figure out a way to stop them. The key to mankind's survival is a towering mecha known as Zearth, but the iron giant's power is not infinite. It gets its strength from the power of life supplied by each teenage pilot. That's a deadly price to pay for someone not yet in high school.