Kasumi, a serious and earnest girl, enrolls in an elite all-girls academy where tradition and refinement reign. One day in the school library, she discovers a letter tucked inside an old book. The letter’s contents are intriguing and mysterious: it expresses a heartfelt desire for a relationship as profound and special as the one described in the book. Thus begins a tale of intrigue and emotion as Kasumi delves into the story behind the letter. Her journey leads her to uncover the history of the school and the bonds of "S"—a term symbolizing deep affection and admiration between young women. Drawn into the allure of these relationships, she learns about the legacy of past students and finds herself questioning her own feelings.