Kang Woo Hyun is brought into the emergency room due to his numerous injuries. He claims that he was hunted by people and his family is missing. Go Dong Gook, a detective nearing his retirement, is dispatched to take care of it. Woo Hyun's testimony sounds insane, but it is too coherent and detailed to ignore.

Detective Bae has fallen from the top of the hospital and died. Dong Guk believes, Detective Bae was murdered by someone. A criminal profiler, Yoon Seo Young, joins the investigation to help Dong Guk but refuses her help. As the search team looks around the mountain, Woo Hyun's testimony is getting more believable. Woo Hyun's close acquittance, CEO Hong Won Tae, visits Woo Hyun in the hospital to check on him. The benevolent and charismatic CEO may have a different face no one knows of.

The search team has found Woo Hyun's son's body. Everyone is shocked by how much the boy's corpse was wrecked by the hunter. As someone with experience in losing a child, Dong Guk gives a piece of advice to Woo Hyun. CEO Hong turns out to be an accomplice to Woo Hyun's case. The difference between psychopaths and regular people are hard to distinguish, just like a 1mm difference.