A serious and earnest girl named Kasumi begins her studies at an all-girls school. While browsing the library, she discovers a letter tucked inside an old book. The letter writer expresses a desire to form a special relationship with the reader, mirroring the kind of intense emotional bond described in the book itself. This invitation draws Kasumi into a secret exchange of letters with a mysterious person she has never met, eventually leading her to form an age-reversed sisterly relationship reminiscent of the Class S tradition with a first-year student named Haruyo. As Kasumi and Haruyo grow closer through their correspondence and eventual meetings, Kasumi learns that their letter exchange is not the first of its kind; previous students, Saira and Mizuki, had used the same book as a conduit for their own hidden connection. The narrative weaves together the present-day relationship between Kasumi and Haruyo with the earlier story of Saira and Mizuki, exploring the enduring nature of affection, the weight of history within the school, and the quiet intensity of girls drawn to the ideal of an all-consuming bond.