Every 100 years, 100 demon children are sent to Earth to compete in a battle to become king of the Demon World. Each demon is paired with a human partner who is given a spell book of a specific color. The human can cast the demon's spells through their book. If a demon's book is burned, they disappear and return to the Demon World. The last demon remaining becomes the king. The manga is oriented around Zatch Bell and his human partner, Takamine Kiyomaro.