A high school girl, Takamine, who can't find what she likes and spends her after-school time aimlessly. One day, she meets Sakura, a student from another school who loves reading, after mistakenly taking each other's books on the train. Through their mysterious after-school exchanges centered around "books," Takamine and Sakura begin to be drawn to each other──. A blue spring of girls nurturing a nameless relationship that is neither friendship nor romance──.