When wondering about which club to join, Yako gets a bit agitated when her mother is brought up. Whilst changing, Nayu spots Yako with a bra her mother brought, who gets riled up when Nayu tells her it is too soon for her to wear one. She tries it on anyway and it ends up shifting during PE. Embarrassed, Yako vows never to wear a bra again. Nayu, with the help of her step-brother Keigo, makes a no-cup for Yako, but she declines it. She reveals she is embarrassed to wear a bra over her flat chest since it might cause others to laugh at her. When Nayu explains to her about her breasts swelling, she finally convinces Yako to wear the bra, which she finds more comfortable. Whilst discussing what she went through to make it, the idea of an underwear club springs to Nayu's mind.
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.
