A boy falls to his death at school, but Ok Chan-mi does not believe that her twin brother, Park Won-seok, committed suicide. Chan-mi transfers to her brother's school, Yongtan High, and meets Ji Soo-heon, who witnessed her brother's death. 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.

Shin Ye-eun, Lomon, Seo Ji-hoon, Chae Sang-woo, Lee Soo-min, Chung Su-bin, Kim Joo-ryoung, Wooyeon, Jin Ho-eun, Yeon Oh, Ok Jin-uk