So-ri, a high school student who is socially excluded at school after standing up for a bullied friend. Since then, she struggles with loneliness and self-doubt. Hoping for a fresh start, she transfers to a new school, only to find herself trapped in another cage. One day, So-ri discovers a letter under her desk from a student who once sat in the same seat, Ho-yeon. The mysterious letters lead her on a treasure hunt across the school, but what will be waiting at the destination?