The third grade D class at a prestigious high school in Tokyo is made up of high-achieving students. Their previous homeroom teacher went insane and quit their job in a short period of time. Students in third grade D class push their homeroom teachers to the limit and watch as they mentally collapse. It is viewed by the students as a game that they enjoy playing. Tsuneo Yoshizawa (Kei Tanaka) appears as the new homeroom teacher in third grade D. Tsuneo Yoshizawa maintains a smile despite being verbally and physically attacked by the students. He never fails to smile. The students' attitude toward Tsuneo Yoshizawa deteriorates, and they devise a plan to assassinate their teacher.