One day, in the library where Sakuta Azusagawa studies, he runs into an unexpected sight: a beautiful high school girl in a bunny outfit. She turns out to be Mai Sakurajima, a famous former actress and fellow student at Minegahara High. His surprise at seeing Mai wear such risqué attire in the library, however, is nothing compared to the shocking realization that he is the only one who can see her!

Reminded of an event from his past, Sakuta offers to help Mai discover the reason behind her invisibility. Despite their initially contentious relationship, the two quickly become close: Mai and Sakuta learn to appreciate each other, and their friendship turns into a budding romance. This further fuels their determination to solve Mai's dilemma.

While the investigation progresses, Sakuta attributes Mai's situation to an urban legend known as Adolescence Syndrome: strange phenomena occurring in youth, unexplained by science and often dismissed as delusion. With few promising results, Mai's predicament worsens to the point where being unseen is the least of her worries. Can Sakuta and Mai solve the mystery of her Adolescence Syndrome before she disappears completely?