Na Bori (Gong Hyo Jin) is a 25 year old woman who desperately wants to be a teacher at the high school she was kicked out of due misunderstanding with her teachers over injuring a group of male classmates. After getting her GED, she rejects job offers from other high schools and works on becoming a teacher at her old high school so she could work side by side with her high school love interest, the art teacher Ji Hyun Woo (Kim Da Hyun). However, the only thing that enables her to get the coveted job is by signing a contract to essentially babysit Park Tae In (Gong Yoo), a troublemaker who attends the same high school. Tae In, a lonely youth, quickly becomes the "jjang", or "king" of the high school, and enjoys playing mean tricks on Bori whose easygoing personality attracts the majority of her students. Noh Jemma (Choi Yeo Jin) is jealous of the relationship between Bori and Tae In, and goes out of her way to get Bori kicked out but she's not the only one against them...