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. Ko Dong Guk, 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.

Even after recovering his memories, Woo Hyun does not disclose everything he knows to the police. To cover CEO Hong and his group’s mistake, they pull some strings to switch out the team in charge of Woo Hyun’s case. Ms. Kim is on the run from both the police and CEO Hong and his group. The situation seems dire but Dong Guk and Seo Young both head over to Woo Hyun’s house for evidence that can unveil the truth.

To provoke and bring CEO Hong out, Seo Young decides to go on the news and claim she and her team are close to catching the culprit behind Woo Hyun's case. Naturally being an ostentatious psychopath, CEO Hong decides to kill them before they get any more evidence. After a fierce brawl, CEO Hong is caught red-handed and gets shot. Although the case is not closed, Dong Guk celebrates what happened. Meanwhile, Seo Young finds something that was in her blind spot due to her prejudice.

After finding out about Woo Hyun’s 1mm difference, Seo Young is gravely injured and sent to the hospital. Dong Guk is trying to lay low and forget about the case after seeing what happened to Seo Young. Meanwhile, Woo Hyun is trying to prove himself to the same group that was hunting him down to join them. Impressed by Woo Hyun’s job of turning the entire country into his hunting ground, they decide to test him out.