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. 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...

Cast
* Gong Hyo Jin as Na Bo-ri
* Gong Yoo as Park Tae-in
* Kim Da Hyun as Ji Hyun-woo
* Choi Yeo Jin as Noh Jem-ma
* Oh Yoon Ah as Chae Eun Song
* Jang Hee Jin as Oh Eun-byul
* Yang Geum Suk as Ji Young He
* Shin Ah as Kang Ri-ping
* Cha Seo Rin as Lee Hye-bin
* Kim Eun Joo as Kim Sun-ah
* Park Ki Young as Pang Soo-hyun
* Jung Ui Chul as Lee Ho-joon
* Jung Kyung Woon as Choi Chang-il
* Do Han as Lee Yoo-jin
* Jo Sang Ki as Nam Sung-ki
* Park In Hwan as Hwang Gap-soo
* Jo Hyung Ki as Dong Chil-hwan
* Kim Yoon Kyung as Oh Joo-yeon
* Lee Yoon Ji as Na Sun-jae (Bo-ri's younger sister)