Mizuno has a rather dubious dream about Keigo. The next day the Underwear Society go on an overnight trip to a beach hotel where Nayu's grandmother used to stay. Mizuno brings her old school swimsuit which the girl's accidentally wreck whilst trying to remove the nametag, so they take her to buy a new one. After rain cancels the swim session, the girl's go to a hot spring, where Hiroki passes out over hearing the girls, and is embarrassed to find that they carried him naked over to his bed. After the girls give Mizuno a makeover, the hotel mistress tells them about Nayu's grandmother. After a trip to the beach the next day, everyone heads back all sleepy, although Hiroki is annoyed that they didn't discuss any club activities the whole time.
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.
