When Nanao was a mere child, she was kidnapped by an Ayakashi. Since then, because of the spirit seal the Ayakashi carved on her body, Nanao has been oppressed by the people in her own clan as the "tainted one."
Nanao was supposed to get married to the young master of the Byakurenji family, but her cousin Akemi stole her position as his bride, and she was forced to wear a monkey mask in order to hide the spirit seal, living a miserable life. Then one day, she meets Yako, the young head of the Benitsubaki clan.
Due to an unforeseen event, Nanao's mask comes off and her face gets exposed, but that only causes Yako to become bewitched by her beauty and high spiritual power... (Source: Manga Plaza)
— Description from AniList community (anilist.co).
