The protagonist, Jo Kang-ja (Kim Hee-sun) was once legendary for being the toughest, most feared troublemaker at her high school in Busan.

Due to her pregnancy in her late teens, she drops out of school and tries to become a responsible mother to her daughter, Oh Ah-ran (Kim Yoo-jung).

Fast-forward to the present, Kang-ja is now 34 years old and Ah-ran is a teenager attending high school.

Unbeknownst to Ah-ran, Kang-ja enrolls in Myungsung High as an undercover student with the false name "Jo Bang-wool" to teach those bullies a lesson.

Kang-ja makes it her mission to put a stop to the school violence, dig up the flaws of the education system, and school corruption at Myungsung High with the help of Ah-ran and the naive, idealistic homeroom teacher Park No-ah.