Chu-Bra!!

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.

One of the male students, Hiroki Komachi, tries to have a word with Nayu, but ends up getting occupied at every chance he gets. Nayu, Haruka and Yako take a look at the Crafts club, but leave when the club's adviser, Tsukamoto, shows disdain to the idea of making underwear. Noticing Nayu is disheartened, Yako suggests that she make an Underwear Club herself. The next day, Hiroki complains to Nayu concerning an apparent rivalry, but is annoyed that Nayu doesn't remember him. Nayu enquires about how to start her club, but is put down again by Tsukamoto. However, after looking at the underwear her grandma gave her, she perks up and starts asking around for members, but has trouble finding any. Hiroki talks to Nayu, suggesting that she should try and put across her vision of underwear for better results.

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.