On the first day of middle school, Nayu Hayama accidentally shows her black lace panties during the opening ceremony. Yako Jingūji and Haruka Shiraishi hear rumours of her doing "enjo-kosai" or prostitution, and spot her talking on the phone about uncomfortable underwear to a man who arrives to deliver her some underwear. Curious, the two follow Nayu to the underwear department, but run off when they are spotted. When Nayu catches up to them the next day, she grabs Haruka's breasts from out of the blue, prompting both Yako and Haruka to keep their distance from her. Yako tries to find undeniable evidence by getting a photo shot under her skirt, but starts to slip down the stairs. Haruka grabs her and Nayu grabs Haruka, though they end up falling regardless. Nayu explains that she is an underwear monitor who tries different types of products, and mentions she was inspecting Haruka because her bra was too small. Although the resulting chaos in trying to get the proper measurements leaves Nayu with a worse reputation than she already had, she, Yako and Haruka become friends.

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.

Nayu explains her situation to Keigo. Hiroki then comes over to Nayu's house to apologise and ask is there's anything he can do for her, inadvertedly offering to help her make an 'Underwear Appreciation Society'. Nayu also manages to convince Yako, Haruka and Mizuno to become members too. They hold the meeting at Nayu's house, where Mizuno is surprised to see Keigo, who was her senpai in high school. Nayu contemplates about love and whether she is adult enough to run a society. Keigo explains to Mizuno about how Nayu's grandmother helped her find confidence in underwear when she was in a bedwetting phase. The next day, Kiyono decides to join the Underwear Appreciation Society.

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.

As the Underwear Society make preparations for the drama club, they are asked to participate in the performance. After school, Nayu runs into Kiyono, who had heard about Keigo's job offer. The next day, Nayu takes on some extra work whilst Kiyono tells Hiroki about Nayu's apparent transfer. He confronts Nayu about this, who asks him to stay quiet until after the festival. On the day of the festival, Tsukamoto confronts Mizuno and Nayu about the society's appearance in the magazine. She asks the play to be cancelled, but Mizuno stands up to her and tells her to hold her judgement until after she's seen their work. However, during her inspection, Nayu's old elementary school teacher comes in a berates Nayu. Thankfully, both the society members and Tsukamoto stand up to him, the latter of which allows the play to take place. After a successful performance, Nayu tells Yako and Haruka that she's transferring as Keigo tells Mizuno the same thing.