Sono Uruwashiki Hito wa centers on an ordinary teenage girl named Koto and her close friendship with Mi, the most famous and seemingly perfect senior at their all-girls academy. Mi is the embodiment of a flawless student, combining beauty with talent, making her the ideal figure within the school's hierarchy.

The peaceful rhythm of the academy shatters after summer break when Mi suddenly vanishes without explanation. As Koto grapples with her friend's disappearance, unsettling rumors begin circulating through the school's corridors, casting doubt on everything Koto thought she knew about Mi. Driven by concern and confusion, Koto begins searching for clues about where Mi has gone and why she left. During her investigation, she stumbles upon a hidden diary.

Inside the diary, Koto discovers a shocking truth about Mi that completely redefines the beautiful, perfect senior everyone admired from a distance. The narrative unfolds as a mystery set against the backdrop of the girls' school, combining the atmospheric tension of a missing person case with the emotional intimacy of a story about friendship and hidden identities. The manga explores how well people truly know one another, especially when their public image is constructed to be flawless. The diary serves as the key narrative device that transforms Koto from a passive observer of Mi's perfection into an active seeker of the truth, forcing her to reconcile the girl she thought she knew with the person revealed in the pages of the journal.