Everything Masato believes about his own life is a transaction. His parents sold him to a government program before he could remember it. The robot they hand him isn't a hero's calling, it's proof that he was never actually chosen, just manufactured for a war that started before he was born. Zeorymer moves like weight and consequence, not spectacle. The psychological damage accumulates faster than the plot, and by the end you understand that the real enemy isn't the organization trying to use him, it's the fact that he has no self left to refuse them.
Masato's ordinary life shatters when a secret government project called Last Guardian takes him and reveals he is no ordinary student. He is meant to pilot Zeorymer of the Heavens, a giant robot. The truth hits home when he sees the people who raised him as his parents accept payment for it. Fifteen years earlier, Hau Dragon, an organization aiming to conquer the world, built eight great mecha, each tied to a force of nature. Before they could be used, their creator Kihara Masaki destroyed all but the leader, Zeorymer, and brought it to the government along with an embryo that grew into Masato. Now Hau Dragon has rebuilt the other seven machines and wants the eighth. Masato and Himuro must pilot Zeorymer against it, though neither of them is quite what they appear to be.
