Major crimes unit detective Seong-jin is a righteous police officer with a wife and a son who needs surgery for his legs. When he arrests Tae-sung on a petty charge, the case takes a shocking turn: the suspect may be a notorious serial killer. As the investigation unfolds, Seong-jin is drawn into a conspiracy and forced to question what is true, what is being used against him, and whether to protect his principles or his family.