Rina Kujo, the homeroom teacher of Class D, Grade 3 at Houlai High School, is a chemistry teacher who has given up on keeping up with the times and the students. On the day of the graduation ceremony, she was pushed down by "a student in the class", and then time went back, and she stood on the podium, and saw the "30 suspects" who would kill her a year later. She made up her "determination" to overturn the classroom and her life, and challenged her students at the risk of her life.