Kiyono reveals she is going to be a model for her mother's underwear collection, but has a problem with her current breast size. Nayu and the others try to help make them bigger, but to no avail. She tries doing the shoot wearing a slightly bigger bra, but it's soon noticed and when her mother confronts her about it, she runs off. Spotting some more comfortable bra prototypes, Nayu decides to decorate them to fit Kiyono's likes better. Kiyono's mother gives them their thanks, and also mentions Keigo's offer to Nayu. After being featured in the magazine as the designers, the Underwear Society are approached by the drama club to produce underwear for their costumes for the cultural festival. When Nayu asks Keigo about his offer, he tells her that he is going to Kyoto.
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.
