Oh Ah Ran (Kim Yoo Jung) is a high-schooler who sticks up for her bullied friend and becomes the victim of bullying in turn. Jo Kang Ja (Kim Hee Sun) is her mom, a former Busan badass who got pregnant in her teens and raised her daughter as a single mom. When Mom hears about the bullying that’s going on in her daughter’s school, she takes matters into her own hands and goes undercover as a high school student to fix the problem.