"S" is a known as a bully caretaker, who avenges her bullied clients—for a price.

However, in reality, "S" is a nerd named Kang Song Ih, with a twin brother named Kang Ah Eul, whom she loves above anything. Ah Eul attends Chungha High School, a famous and expensive school, but over winter break, he attempts to commit suicide. Song Ih, determined to understand why her brother tried to kill himself, attends Chungha High School, pretending to be Ah Eul—uncovering the reasons why Ah Eul was driven to suicide, including one mysteriously handsome but "bad" guy named Suh Ga Yoon.