The old teaching building of Class 7, Grade 3 hides the taboos of the evening self-study from ten years ago. Do not count the number of people in the last row; someone tells you not to look back after ten o'clock; if someone asks 'what time is it', you must not make a sound. Zhang Lei returns to the classroom on the day of the ten-year memorial and discovers a seventh person appearing out of thin air in the last row. The yellowed roll book, the scorched red armband, and the recurring bell step by step uncover the truth of the fire back then—the deserter you thought was actually the only one who went to ask for help; the survivor you thought never walked out of that classroom.