Fumino, the fiancée of Yuu, the eldest son of a 150-year-old sake brewery, needs her strict father's approval to marry into the prestigious family. Fumino, with her angelic smile and pure demeanor, seems like the ideal bride... but she harbors a secret: she's actually a legendary 3-time consecutive No. 1 courtesan known as "Karen-chan." Her true identity must remain hidden. She teases Yuu by emphasizing her chastity and refusing sex until marriage. However, Yuu starts to suspect Fumino when her incredible skills unconsciously activate. Yuu's father, concerned, hires a detective to investigate Fumino. What will happen next??