A world of sorcery, without knowledge of magic. One mercenary, who is called "beast-fallen" (half beast, half human), was being hunted down by witches for his head is said to be an excellent instrument for their sorcery. But he can't get caught before his dream comes true...becoming human being. The one day, he was saved by a witch in a forest. She says that she is looking for "Grimoire of Zero", a book about magic which can destroy the world. Her name was Zero...and she was the great witch who found magic. The beast agrees with Zero to go with her...thus the journey begins.