After losing her husband to serial killer, police detective Lee Hwa Sun makes her way to Muji city in order to bring the murderer in. But unlike what she is hoping to find, she runs into an enormous sinkhole in the middle of the mountains.
The news of the sinkhole spreads throughout Muji and more people come to look. However, things go wrong as they get infected by the black smoke and start attacking each other.
Hwa Sun and Do Yoon make it to the safety of Muji High School. However, they get confronted by Mr. Choi, who thinks they are a threat to everyone inside. At Muji Hospital, Tae Han attempts to rescue survivors on the fourth floor.
Tae Han continues to look for the survivors in Muji Hospital. However, he gets a strange feeling something is following him. Meanwhile, at the school, Hwa Sun and a few risk their lives to fetch medicine from the nurse's office.
Do Yoon and the survivors at the school wait for Hwa Sun to return, but they soon find out Jin Seok had his way. Under the influence of the monster, Jin Seok takes down the barricade to the school, allowing the mutants to enter.
Hwa Sun and the rest escape from Muji High School, while the shaman controls the people in the hospital. As Hwa Sun makes her way to the Muji Hospital, the monster within the smoke takes control of people one by one.
Tae Han rushes to the hospital after Hwa Sun tells him about her vision. When he gets there, he gets caught up in the chaotic ritual of the shaman and joins the officers as part of the sacrifice.
Hwa Sun confesses to the group that she inhaled black smoke. But what shocks them more is the monster she thinks is hiding inside one of them. Meanwhile, more survivors join the shaman, and a doctor takes matters into her own hands.
After reading the monster's message, Hwa Sun, Tae Han, and Soon Il go to Muji Chemical to rescue Do Yoon. Although they think it's a trap, they risk their lives to end it once and for all.
