Haruka feels down when the boys start teasing her for her big breasts, even ranking her #1 in a 'biggest breasts' chart. She and Yako visit Nayu's house, where Nayu asks them to be models for a fitting session to gain awareness for the underwear club. Whilst Yako agrees, Haruka is a little worried about it. Nayu later tells them how she came to live with Keigo after her parents died. Haruka tells Yako about the boys' ranking, but says she will bear with it for Nayu's sake. Nayu manages to get permission to use an empty classroom after educating the teacher, Tamaki Mizuno, about her underwear. The session gets a great turnout and Haruka is surprised by the kind response to her modelling. Hiroki tries to stop some peeping toms, but both he and the girls end up getting lectured by Tsukamoto, saying the underwear club would never come to be. Afterwards, the girls stumble upon some underwear that even Nayu has never seen before, and are teased by another student, Kiyono Amahara.
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.
