The story follows detective Ko Moo-chi (Lee Hee-joon)'s attempt to catch a psychopathic serial killer, after witnessing a horrific incident as a child and dealing with his demons from the trauma. Along with his rookie partner Jeong Ba-reum (Lee Seung-gi), he attempts to unearth the truth behind psychopathic behaviors and find answers about the genetic predispositions towards psychopathy — and how one might act on the knowledge in the event that such predispositions could be detected.