The letters Nanako writes to her brother become a confession booth, then a morgue. What starts as ordinary school friction hardens into something genuinely dark: girls destroying each other with precision, one friendship at a time, while the Sorority watches and smiles. This is a character study that doesn't flinch when characters break.

Sixteen-year-old Nanako Misonoo enters Seiran Academy, a renowned private school for girls, expecting a bright future. As an ordinary middle-class student, she lacks the wealth, grace and talent of her peers. She is then chosen to join the Sorority, the school's most elite and envied club. Her selection provokes jealousy from classmates, above all the fierce Aya Misaki, and she faces cruel rumours and constant social trials. She recounts her days in letters to her former cram school teacher, Takehiko Henmi, whom she fondly calls big brother. She also leans on four close friends: childhood friend Tomoko Arikura, the sociable but unpredictable Mariko Shinobu, troubled musician Rei Asaka and athletic tomboy Kaoru Orihara. The result is a psychological drama about bullying and the hardships of school life.