The world is plagued by an ongoing conflict between humans and witches, and in the midst of it all, there exists a group known as the "beastfallen." These are humans who have been cursed, and as a result, they possess the strength and appearance of various animals. Due to their savage nature, beastfallen are often avoided or feared by both humans and witches. Many of them resort to becoming sellswords, making their living by hunting witches. Despite the animosity between the two races, a whimsical witch by the name of Zero seeks the aid of a beastfallen, who goes by the name of "Mercenary," to act as her protector. Alongside Albus, a young magician, they set out on a quest to locate the Grimoire of Zero: a powerful spell book that could have catastrophic consequences if it falls into the wrong hands. As they journey together, the group bonds and Mercenary's innate kindness and empathy begin to reveal themselves, allowing him to align with both humans and witches alike.