This story takes place in the Ulstrauss Empire located in the Western Continent. Here, humans and demons are at odds, fighting day in day out. The female lead, Lady Ninita, is the only child of the Marquis, and has been pure and innocent since young. Additionally, she possessed a high aptitude for exterminating demons. Everyone looked forward to seeing her train to become the Saintess, who appeared once in a millennium. However, due to a strange combination of circumstances, she and the pure-blood demon Wolf King, Geoff, fell in love with each other instead.