A hard-hitting detective's live is turned upside down when the woman he loves is the latest victim of a serial killer.
Episode 1.1: When the police discover skeletons on the train tracks at Mukyeong Station, Detective Do-won heads there while chasing a key suspect. (S1, ep 1)
Episode 1.2: Do-won confesses that his father killed Seo-kyung's father. Later, Seo-kyung discovers her mother's jewellery on the recently-discovered bodies. (S1, ep 2)
Episode 1.3: Seo-kyung dies at the hands of the Mukyeong serial killer. Do-won realises that the killer is the same culprit as from the murder case 12 years ago. (S1, ep 3)
Episode 1.4: Do-won arrives in World B and meets the Seo-kyung of this world. He discovers the other Do-won is on the run as a prime suspect in a murder. (S1, ep 4)
Episode 1.5: 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. (S1, ep 5)
Episode 1.6: Jin-woo is the prime suspect in World B, and Do-won tries to figure out what has happened. Seo-kyung seems to have a dark past in World B. (S1, ep 6)
Episode 1.7: Do-won finds traces of the killer on the body of Jo Young-ran. Seo-kyung and Do-won begin tracking the culprit of the 12-year-old case once again. (S1, ep 7)
Episode 1.8: As they investigate Sung-wook's past, Do-won and Seo-kyung find themselves at another dead end. Meanwhile, the two Do-wons meet face to face. (S1, ep 8)
Episode 1.9: 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. (S1, ep 9)
Episode 1.10: Do-won finds out who the real killer is. However, the killer is one step ahead of everyone else. (S1, ep 10)
Episode 1.11: After getting shot by his suspect, Do-won returns back to his world. In World B, Seo-kyung discovers an unexpected clue. (S1, ep 11)
Episode 1.12: Do-won saves Mi-sook, but she points a gun at Seo-kyung. Thinking he keeps on putting Seo-kyung in danger, Do-won makes a difficult decision. (S1, ep 12)