In the past, a mother oni gave birth to a human-like baby who held a sword instead of a horn. This child, known as "Onikirimaru" or Ogre Slayer, is completely ogre even though he looks human. His goal is to become a true human only after all ogres are eliminated from the human realm. As a result, he sets out on a mission to eradicate all ogres, and he still pursues this mission in modern-day Japan.