Kasai Koharu, a beautiful girl with an exceptional sense of style, is assigned as the new replacement teacher after the main high school instructor was hospitalized due to relentless bullying from the students. <br><br> Though she appears airheaded and constantly wears a bright smile, the students continue to mistreat her, unaware of the dark secret that their teacher, Kasai-sensei, is a real psychopath. <br><br> “The reason I became a teacher is to bring happiness to my students.”