Asano Sumhitomi is an ordinary student who has been studying in An'an, Japan for two years. However, one day two years later, she suddenly received a final letter from the suicide victim Mu Yun Feixue! This made Asano, who wanted to study hard, feel scared and uneasy. It turns out that Asano was Mu Yun's friend during his lifetime, but why did she receive Mu Yun's last letter two years after his death? While Asano was confused, he discovered that the deaths of his cousin Zichuan and Mu Yun were mysteriously related, and the cousins died within a week of Mu Yun's death... This series of time was pulling each other in the dark... When Asano made up his mind to investigate the case with his friend Ji Chuan, he discovered...