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.

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.

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.

After Kiyono joins the society, the girls become aware of her habit of teasing what's under her skirt in front of boys. She flips up Yako's skirt and makes fun of the underwear, only to show some embarrassment when she flips hers back. The girls are surprised when Kiyono takes off her underwear before leaving. The next day, a moral exam is held and Kiyono is told off for wearing makeup. In the meeting later that day, Nayu brings a deconstructed bra, but Kiyono shows disinterest. She then insults Mizuno, particularly about her femininity and bra choice, and shows everyone catalogues of more kinky underwear. Kiyono argues with Nayu over looks vs comfort, but feels some embarrassment when the topic of who she loves is brought up.

Hiroki's newfound knowledge of underwear is affecting both his concentration and his reputation. Nayu gets a mysterious 'love' letter in her shoebox. Hiroki gets annoyed and decides to quit the underwear society. On his way home, he runs into Keigo, who tells him about Nayu's efforts to make the society more male-friendly. Nayu's confessor had apparently put his letter in the wrong shoe box, and Hiroki apologises to Nayu and gives the society another shot.

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.

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.

Kiyono reveals she is going to be a model for her mother's underwear collection, but has a problem with her current breast size. Nayu and the others try to help make them bigger, but to no avail. She tries doing the shoot wearing a slightly bigger bra, but it's soon noticed and when her mother confronts her about it, she runs off. Spotting some more comfortable bra prototypes, Nayu decides to decorate them to fit Kiyono's likes better. Kiyono's mother gives them their thanks, and also mentions Keigo's offer to Nayu. After being featured in the magazine as the designers, the Underwear Society are approached by the drama club to produce underwear for their costumes for the cultural festival. When Nayu asks Keigo about his offer, he tells her that he is going to Kyoto.

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.

Haruka feels betrayed by Nayu's announcement and stops being friends with her. That same night, Mizuno's apartment catches fire. These events leave everyone in a negative mood, and Nayu's texts to Haruka fall upon deaf ears. Later on, the girls help Nayu with her move, but Haruka doesn't show. After Yako pleas with her to make her final goodbye, Haruka finally meets up with Nayu the next day and the two finally apologise. Haruka, Yako and Hiroki see off Nayu as she heads to Kyoto, whilst they keep up the Underwear Society. However, Nayu later comes back after Keigo and Mizuno make arrangements for her to stay at school.