Yako gets annoyed when her childhood friend, Kota, who insulted her flat chest when she tried changing in front of him. Haruka feels some tightness in her chest, which Mizuno deducing that she is starting to develop feelings towards Hiroki. As the girls head off to a summer festival, Kiyono explains to Yako that Kota felt hurt because he wasn't being registered as a member of the opposite sex. With some encouragement from Hiroki, she goes to apologise. When Haruka accidentally gets a goldfish down her cleavage, she feels the tightness again when Nayu adjusts her bra. Nayu recognises from her underwear that Haruka seems to be in love with someone, though she doesn't get round to saying who. After Nayu helps a lost child, Keigo receives a mail that he has been chosen for the Kyoto Office.
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.
