It centres on a group of survivors who must fight for their lives against mutants – people who have inhaled a mysterious dark smoke from a sinkhole. Lee Hwa-Sun (Kim Ok-Vin) is a detective with the regional investigation unit of the Seoul Metropolitan Police Agency. She receives a phone call from the person who killed her husband, telling her to go to Muji City. Lee Hwa-Sun then travels to Muji City. There, people have inhaled a mysterious dark smoke and turned into monsters. The residents of Muji City are terrified. Lee Hwa-Sun fights to save the people of this place and also pursues her husband’s killer. Yoo Tae-Han (Lee Joon-Hyuk) has lived in Muji City all his life. He once worked as a detective, but had to leave due to a scandal caused by a misunderstanding; however, he is proud of his time in the police force. He now works as a driver for a demolition company. He still possesses a sense of justice, whilst also being down-to-earth and a joker. Yoo Tae-Han must now face his city, which has descended into chaos due to the dark smoke that turns people into monsters. He joins forces with Lee Hwa-Sun to save the people of Muji City.