Thirteen-year-old Aonuma Aira lives a colorless, uneventful middle school life. He is deeply withdrawn, socially anxious, and convinced he has no presence at all—so much so that he feels he cannot even be bullied. His world changes abruptly when his father remarries, and he gains a new stepsister: Mizusai, an energetic high school girl with a flamboyant gyaru style. Mizusai is everything Aira is not—outgoing, bold, and unapologetically fashionable. She immediately notices Aira's gloomy appearance, especially his overgrown bangs that hide his face, and decides to take matters into her own hands.

Mizusai's hobby is "decorating" people, and she enthusiastically treats Aira as her latest project. She cuts his hair, applies makeup, and dresses him up, transforming him into a stunningly cute girl. Aira is startled by his own reflection but also begins to gain a flicker of confidence he never had before. The premise sets up a comedy-driven dynamic where Mizusai cheerfully directs Aira's makeovers while Aira, reluctant but curious, becomes her toy in more ways than one.

The setting is a contemporary Japanese town, shifting between the home the two now share, their separate schools, and local spots like the aquarium. Aira's mother and Mizusai's father are present but mostly in the background, leaving the step-siblings to navigate their odd new relationship largely on their own. Aira's classmates and school life gradually enter the picture as he tentatively steps out of isolation.

As the story progresses, two notable narrative arcs emerge. In the school sports festival arc, Aira watches the cheerleading practice from afar, feeling left out of the group spirit. Encouraged or pushed by Mizusai, he ends up performing as a cheerleader himself, a high-anxiety but liberating experience that forces him to engage with others. In the volume 3 arc, Mizusai's birthday approaches, and Aira plans a trip to the aquarium to celebrate. He tries to dress and act manly to impress her, but the day does not go as planned, leading him to feel inadequate. Mizusai responds with reassurance, telling him he does not need to force himself to be someone else. Through these shared moments, Aira begins to recognize a budding romantic feeling toward his stepsister, even as Mizusai's own intentions remain cheerfully ambiguous.