From lecturing, meetings with students' parents, self-defense to first-aid treatment, Ma Yeo-jin masters everything and strives for perfection in every aspect. She joins Sandeul Elementary School as a homeroom teacher of Class 6-3 after her two-year hiatus. With a poker face, Yeo-jin teaches her rebellious students using an unorthodox and unique method, and she tells them the inconvenient truth.