Because of a letter written ten years ago, Tetsuhiko went back to the past. The letter paper in his hand had yellowish traces and the black ink was slightly faded. Because of this letter, Tetsuhiko went back to the past without any preparation, back to an era when computers were not very common and iPhones were unknown. Maybe he was panicked at first, but fortunately he met Xia Shiguang. Later, Zheyan gradually got used to this kind of life. Just as his relationship with Shiguang was getting closer, Zheyan felt that another soul in his body was awakening. Who is Shiguang, and why does he become so strange when he is with him? What did that letter say? What does time want?