A boy falls to his death at school, but Ok Chan Mi does not believe that her twin brother, Park Won Seok, committed suicide. She then transfers to her brother's school, Yongtan High, and meets Ji Soo Heon, who witnessed her brother's death. During that time a "hero," who avenges bullied students, appears at Yongtan High. Chan Mi speculates that it may be connected to her brother and starts looking for this hero.