Season 1

After losing her husband to serial killer Lee Soo Yeon, 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. Black smoke arises from this mysterious sinkhole and the people of Muji are left clueless about what it is.

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. Not knowing how it is triggered, but a large amount of smoke ascends from the hole and slowly covers Muji city. Hwa Sun and Tae Han go separate ways for their own objectives, but they both fight their way through the chaos.