In Toson, a disaffected university student named Leonardo Yohann finds the "Angel Book", a mysterious black angel book that can kill anyone as long as the user knows both the target's name and face. Initially terrified of its angel-like power, Leonardo considers the possibilities of the Angel Book's abilities and kills high-profile Toson criminals, then targets international criminals. Five days after discovering the angel book, Leonardo is visited by Rico, an "angel" and the Angel Book's previous owner. Rico, invisible to anyone who has not touched the angel book, reveals that he dropped the angel book into the human world out of boredom and is amused by Leonardo's actions