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.