The drama tells the story of a monster who hides in a sinkhole that sucks up souls. While on a quest to find this creature, the characters find themselves turning into equally cruel monsters, and in turn they discover what makes us human and how precious courage and devotion are to human life.
Kim Ok Bin is in talks to play homicide detective Lee Hwa Sun. Due to her own goals, Lee Hwa Sun is unable to protect those closest to her and suffers from extreme remorse and helplessness. After eventually trying to take her own life, she witnesses a murder and sets out to get revenge.
Lee Joon Hyuk is in talks to portray former commando sergeant Yoo Tae Han, who grew up carrying the burden of his family. As soon as he turned 20 years old, Yoo Tae Han enlisted and was dispatched to Somalia. He is simultaneously a dull and strong person, who has little to say and speaks more with his gaze than his words.