Hiroki's newfound knowledge of underwear is affecting both his concentration and his reputation. Nayu gets a mysterious 'love' letter in her shoebox. Hiroki gets annoyed and decides to quit the underwear society. On his way home, he runs into Keigo, who tells him about Nayu's efforts to make the society more male-friendly. Nayu's confessor had apparently put his letter in the wrong shoe box, and Hiroki apologises to Nayu and gives the society another shot.
The story revolves around Nayu Hayama, who embarrasses herself on the first day of middle school by accidentally showing her adult panties. As two other students, Yako Jingūji and Haruka Shiraishi, hear rumors of her engaging in "enjo kōsai", or compensated dating (which often is viewed as being close to or the same as prostitution), they investigate. They soon learn that Nayu is an 'underwear monitor' who tests new underwear products, and has great insight on what underwear people should wear. Nayu hopes to help everyone get through the vital stage of their life by opening an underwear club.
