Gu Sera is the top troublemaker at Gangsan High, the most infamous high school in the country. When she was a child, she secretly had a crush on Lee Seojoon, the older man living in her boarding house. Now, he has come to Gangsan High for his teaching internship, where he must deal with constant difficulties caused by the unruly students. To protect her crush, Gu Sera resolves to turn the nation's most problematic high school into a model institution.