The world of sword and magic where the hero and the demon king fight. Fell — a demon girl, sets foot on the land where humans live with her master the demon king, with a purpose of establishing a friendly relationship with them. Under such circumstances, despite possessing an extraordinary power as a demon race, Fell is continuously being pushed around by the humans around her. Later in the distant future, the diary that Fell wrote during that time will be found in the deepest level of the biggest dungeon in the world, and become a controversy. There are those who call it the holy scripture, while the others call it the forbidden book. This is a story about a demon girl called Fell and her diary, the story of laughter and tears.