Haunted by the thought that his drunkard father killed another man before also dying on the same day and not paying for his crime, Seo Do Won lives his life trying to secretly repay Han Seo Kyung for his father’s sin while hiding the truth. He is now a detective who overworks to escape his guilt, and she is a prosecutor who is still looking for her father’s killer.
One day, DoWon stumbles upon corpses buried in an abandoned, old train station. Classifying it as a serial murder case, Do Won and the police start investigating. However, the case becomes weird when a dead victim turns up alive and Seo Kyung is killed along with the discovery of a new corpse. Realizing that the mysterious train that shows up in the abandoned station is a portal between parallel universes, Do Won travels to the other world to track the serial killer with the help of the other Seo Kyung.