A Japanese-American girl named Mizuki Ashiya becomes so inspired by watching high jumper Izumi Sano on television that she decides to move to Japan to meet him. The only problem is that Sano attends Osaka Gakuen, an all-boys boarding school. Undeterred, Mizuki cuts her hair short, binds her chest, and disguises herself as a boy to enroll. Her secret mission is not just to get close to her idol but to convince the talented but disillusioned Sano to return to competitive high jumping, which he quit after a traumatic accident.

The story is set within the competitive and chaotic world of Osaka Gakuen, where students are divided into three distinct dormitories. Mizuki is placed in the second dormitory, home to many athletes, and through a stroke of luck, she becomes Sano's roommate. However, her plan nearly falls apart on her first day when the perceptive school doctor, Hokuto Umeda, discovers her secret. Fortunately, Umeda agrees to keep it, becoming her reluctant ally and confidant. More significantly, Sano himself also accidentally discovers that Mizuki is a girl. Rather than exposing her, he quietly protects her secret, creating a unique and intimate bond between them as they share a room.

As Mizuki navigates the daily hurdles of dorm life, athletic competitions, and school festivals, she forms a close circle of friends. Chief among them is her energetic and boisterous roommate, Shuichi Nakatsu, a soccer player who develops a serious crush on Mizuki. Believing her to be a boy, Nakatsu becomes deeply confused about his own sexuality, leading to many comedic moments as he tries to understand his feelings. The central romantic tension grows as Mizuki remains focused on helping Sano, who slowly thaws from his brooding exterior, while Sano must hide his growing affection for her.

A major narrative arc involves Sano overcoming his psychological block in high jumping, which stems from an accident in which he injured himself while saving a friend. Mizuki’s unwavering belief in him is the catalyst for him to confront his past and begin training again. This journey pits him against his rival, Kagurazaka, and forces him to rediscover his passion for the sport. Meanwhile, Mizuki's disguise is constantly threatened by situations involving sports physicals, hot springs, and her own developing feelings, leading to a story that balances romantic comedy with sincere drama about friendship, perseverance, and identity. The anime chronicles these events as Mizuki tries to keep her secret while her presence changes the lives of everyone in the all-boys academy.