Constance Grail is the sole child of the distinguished Grail family, which upholds the motto "Be you sincere." At 16, she personifies this principle. Constance agrees to an engagement to help her father settle his debts, despite her lifelong hope to marry for love. Regardless, she decides to proceed with the marriage for the family's benefit. At a ball hosted by another noble, Constance encounters a mysterious, captivating woman lingering at the room's periphery. She urges the woman to join the ball before losing her from view. Shortly thereafter, her fiancé's lover confronts her with theft accusations. Facing reputational ruin, Constance begs for assistance. In response, she is possessed by the woman she met earlier, revealed to be the ghost of Scarlett Castiel, a notorious villainess. Through Scarlett's sharp wit and intimidation, Constance is rescued, and her engagement is annulled. In exchange for her help, Scarlett recruits Constance to investigate the events leading to her execution 10 years ago. Although charged with attempting to poison the crown princess, Scarlett insists she never committed such an act. Constance agrees to seek the truth, though her efforts will be watched by those who once knew Scarlett and may prefer the truth to stay hidden.