Do Won and Seo Kyung both lose their fathers on the same night 12 years ago. In the present day, Do Won is a police officer and Seo Kyung a prosecutor. The police discover several skeletons on the train tracks at Mukyeong Station, which has been closed for five years. Do Won heads to the station while chasing a key suspect.

Do Won finds out that the culprit who has been covering up Seok Min Jun’s crimes was Mi Sook. As both Do Won and Seo Kyung wait for Train 8210 to arrive, it suddenly stops running, separating the two in different worlds. Meanwhile, Do Won chases down Mi Sook in World A, and Seo Kyung tracks down Min Jun’s last target in World B.

Do Won finds out who the real killer is. However, the killer is already ahead of everyone else. Min Jun turns himself in only to fool the team as they don’t have the corpses to prove his crime. The squad tries everything to find ways to back Min Jun’s statement and lock him in prison.

Seo Kyung dies at the hands of the Mukyeong Serial Killer. Do Won realizes that this is the same culprit from the murder case 12 years ago. However, the more he investigates, the more mysterious things seem to get. Just as he is falling deeper into sorrow over losing Seo Kyung, Do Won sees someone at Mukyeong Station.

Jin Woo is the prime suspect in World B, and Do Won tries to figure out what has happened in this world. Seo Kyung seems to have a dark past in World B. Do Won tries to console Seo Kyung, but she doesn’t want Do Won to get closer. Jin Woo in World B did not have Do Won to stop him from his wrongdoings.

Jae Chul and Do Won of World B die in a mysterious car accident, and Seo Kyung finds out that the living Do Won is from another world. Seo Kyung feels confused about Do Won’s claim that there are two parallel worlds. Meanwhile, Do Won finds traces of the real culprit on the dead Do Won’s body. He is shocked to find out who the real culprit is.

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Do Won confesses to Seo Kyung that his father was the one who killed Seo Kyung’s dad 12 years ago. Seo Kyung feels shocked and betrayed. Meanwhile, Do Won chases the suspect of the Mukyeong Station Serial Murders, and Seo Kyung discovers her mother’s stolen jewellery from 12 years ago on the recently discovered corpses.

Do Won arrives in World B and meets the Seo Kyung of this world. The other Do Won is supposedly on the run after being named a prime suspect of the murder of a drug broker. He is confused about whether or not the other Do Won really is the culprit. Meanwhile, he is shocked to meet someone unexpected in this world.

The Seo Kyung of World B transfers to Mukyeong Police Station. Do Won of World A starts his search for the victims of his last case before coming to World B, the Mukyeong Station Serial Murder. Meanwhile, a new missing person case comes into Mukyeong Police Station, and Do Won runs into an unexpected person while chasing down a key suspect.

Sung Wook falls off of a building right before Do Won and Seo Kyung’s eyes. As they continue to investigate Sung Wook’s past, they find themselves at yet another dead end. Meanwhile, the two Do Wons end up meeting face to face. As they both continue searching for the real culprit, Do Won’s father tries hypnosis to try to bring back his memories.

Do Won finds traces of the real killer on the body of Jo Young Ran. Seo Kyung and Do Won begin tracking down the real culprit of the 12-year-old case once again. Meanwhile, Do Won discovers something about Seo Kyung’s past during his investigations. Later on, Seo Kyung confronts Sung Wook.