Detective Do-won has feelings for prosecutor Seo-kyung but after hearing about her past, he begins to ignore her. While Do-won is chasing after a suspect of a case, he faces another case of murder at Mugyoung station.

While investigating the fifth victim of the case of murder at Mugyoung station, Do-won senses something odd and something unexpected occurs.

Do-won discovers clues but they all seem to be irrelevant to the case. He runs back to Mugyoung station but rather he faces an entirely new world.

Do-won meets Seo-kyung apart from the present world and realises that he himself also exists in that world. Unlike the present world, Do-won is being chased for murdering Jin-sung.

Do-won realises that he, in the present world, exists for Do-won in the other world. As he straightens out the mixed-up situation, he finds out about the truth behind the death of Seo-kyung's father.

Jin Woo is the prime suspect in World B, and Do Won tries to figure out what has happened in this world. Seo Kyung also seems to have a dark past in World B.