Constance Grail is the only child of the renowned Grail family, whose motto is "Be you sincere," and at 16, she is the living embodiment of this motto. Constance becomes engaged to help her father pay off his debts, although she had always hoped to marry for love. Nevertheless, she resolves to go through with the marriage for the good of the family.

During a ball hosted by a fellow noble, Constance meets a mysterious, alluring woman hovering at the edges of the room. She encourages the woman to enter the ball before losing sight of her. Not long after, her fiancé's lover confronts her with accusations of theft. Facing the ruin of her reputation, Constance pleads for help. In response, she is possessed by the woman she met earlier, who turns out to be the ghost of Scarlett Castiel, a famed villainess. With Scarlett's quick wit and intimidation, Constance is saved, and her engagement is annulled.

In return for her aid, Scarlett enlists Constance's help in investigating the circumstances leading to her execution 10 years prior. Although she was charged with attempted poisoning of the crown princess, Scarlett claims she never did such a thing. Constance agrees to uncover the answers, although her efforts will be scrutinized by the figures who once knew Scarlett and might prefer the truth remain buried.