The opening episode commits fully to a nightmare that never explains itself, and Gai spends the entire series trying to understand a demon grafted into his own arm that thinks like a separate person. The fusion mechanic isn't just a power system, it's possession, compromise, and the terror of losing control of your own body presented as visceral horror. What starts as ambitious fractures into genuine incoherence by episode seven, but those first five episodes grip you with a specificity most action shows never risk.

Gai is found as a newborn in a forest beside a woman who died there, and the swordsmith Amon takes him in. Years later, as Amon's apprentice, Gai loses his right arm in an accident while forging a sword. Amon crushes the demon sword Shiryu and reshapes it into a new arm for him. Now Gai can fuse with the weapon on his arm, but he must learn to rein in its violent urges as he is pulled into battle after battle. Meanwhile, another cursed blade, Zsoltgewinn, slips out of the hands of the Shoshidai, the group that gathers such artifacts, and leaves a bloody trail behind it. The result is a supernatural action ONA about a boy made one with a demonic weapon.