Joon-ki (Jang Hyuk) was once a former Rugby football athlete and is currently a popular physical education teacher at an all-girls' high school. His pregnant wife Seo-yeon (Sunwoo Sun) is due to give birth to their first child imminently. Although he's used to playful advances from pubescent students, things take a dangerous turn when Young-eun (Jo Bo-ah) falls hard for her teacher and daringly confesses her feelings for him. Amid growing rumors, he is soon consumed with guilt and attempts to end the relationship. However Young-eun's pure crush slowly turns to obsession. She starts to regard everyone related with Joon-ki as obstacles, and the farther he keeps her away, the bolder her obsession and madness grows.