Jo Kang-ja was once legendary for being the toughest, most feared troublemaker at her Busan high school. When she gets pregnant in her late teens, she drops out of school and tries to become a responsible mother to her daughter, Oh Ah-ran. Fast-forward to the present, and Kang-ja is now in her thirties and Ah-ran is a teenager. The two have a contentious relationship, with Ah-ran ashamed of her quick-tempered, foul-mouthed, sashimi knife-wielding mother for the latter's unrefined ajumma manners.

When Ah-ran befriends the class outcast, it makes her the target of the school bullies who make her life hell; yet she's too proud to tell her mother what's happening. When Kang-ja finds out that her daughter is getting bullied, she decides to take matters into her own hands. Unbeknownst to Ah-ran, she enrolls in her daughter's high school as an undercover student with the false name "Jo Bang-wool" to teach those bullies a lesson. But, she learns that there are bigger, darker problems within the educational system, and Kang-ja makes it her mission to put a stop to the school violence, with the help of Ah-ran and naive, idealistic homeroom teacher Park No-ah.