Ma Lian is a newly hired subway maintenance worker. Because his colleagues took leave, he was forced to take over the maintenance work of the last subway. When he was working the night shift for the first time, he heard a woman humming in the carriage. He turned around and caught a glimpse of a figure wearing a blue cheongsam. He was then dragged away by the old security guard - the security guard warned him "don't look back" and revealed that the missing workers had all turned back.
After that, Ma Lian discovered that every time he worked the night shift, a woman in cheongsam would wait for him at the door of the carriage, and there would be knocking sounds in the tunnel, like someone digging into the wall. He secretly checked the files and discovered that there was a tunnel collapse accident on Line 1 20 years ago, and seven workers were buried. The engineer in charge of the construction at that time was the father of the old maintenance worker who is now missing.
In order to find out the truth, Ma Lian boarded the last subway again with the "talisman" (a rusty copper coin) given by the security guard, only to find a cracked soul-crushing mirror in the tunnel - what was reflected in the mirror was not him, but the workers who were buried 20 years ago. At this time, the cheongsam woman suddenly rushed towards him. Ma Lian remembered what the security guard said, "find the fragments of the soul-suppressing mirror to mend the mirror." He resisted not looking back and groped for the fragments in the dark, but he heard his father's voice from behind, asking him to "look back"...
