The Liturgical Calendar is now in year 526. The world was aware of the existence of witches and their practice of the infamous art of magic. Despite this, the rest of the world was unaware of the study of magic. Our protagonist is a half-human, half-beast mercenary, dubbed "fallen monsters" by humans. He had always wished to become a human, but one day he met a witch who would change his life forever. "Do you want to take on a human form? Then be my escort, mercenary!" the witch said, introducing herself as "Zero" and explaining that robbers had stolen a one-of-a-kind magical tome from her cave. The grimoire, dubbed "The Book of Zero," was said to contain significant magical information that could be utilized to bring the world to an end with ease.