Lee Hwa-Sun (Kim Ok-Vin) is a detective at the regional investigation unit of the Seoul Metropolitan Police Agency. She receives a phone call from the person who murdered her husband. Lee Hwa-Sun then travels to Muji City. There, people inhale a mysterious dark smoke and turn into monsters. The residents in Muji City become terrified. Lee Hwa-Sun struggles to save people there and she also chases after her husband’s murderer.
Meanwhile, Yoo Tae-Han (Lee Joon-Hyuk) has lived in Muji City all of his life. He once worked as a detective, but he had to quit because of a case. He now works as a wrecker driver. His still has his strong of justice, while he is also easy going and a joker. Yoo Tae-Han now faces his city falling into chaos because of the dark smoke that causes people to turn into monsters. He joins Lee Hwa-Sun to save people in Muji City.