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.

Now the children know the truth: their opponents are human beings, just like them. Despite going into his battle with mixed feelings, Kirie finds one more chance to surprise his fellow pilots. Komo follows after him to what seems like an easy victory...until the enemy's tentacles threaten to overwhelm her.