The lonely postman {Chen Mo} in a remote mountainous area daily delivered ordinary letters to the mountain people, until a strange letter signed "No. 13, Underworld Road" appeared out of thin air. The recipients of these letters are all deceased who have been buried for many years. Every time a letter is sent, the recipient's grave will be dug up that night, and the body will disappear mysteriously. In panic, Chen Mo opened an unsent letter, which contained only five words - "It's time to pay back what you owe me." Immediately afterwards, a dark figure in a black trench coat began to follow him day and night. The face was exactly the same as that of a colleague who died due to his fault ten years ago. The guilt from the past surged up, and Chen Mo suddenly realized that the letter sent by the dead soul was fake, but the revenge through his hands was real. Back then, his colleague uncovered the secret of a tomb-robbing gang in the mountainous area and was disguised as an accident to silence him. Now, the spirit of the dead uses the letter as a blade and forces him to become the "messenger" of revenge.
