A serial killer named Headhunter is terrorizing the country but is hiding in plain sight. Daniel Lee has identified a gene that can predict whether the baby is a psychopath 99 percent of the time.
Decades into the future we see what became of the Go brothers and the headhunter. Bo Reum is at the prison entertaining prisoners with a magic show when he gets a devastating surprise.
Ba Reum realizes that Chi Kook's watch was found on a new victim and notifies Moo Chi. Bong Yi's grandmother sees something she shouldn't and is terrified by her finding.
Moo Chi feels that something is suspicious about Sung Yo Han, but he can't find any evidence yet. A boy gets kidnapped and Moo Chi gets a call from the kidnapper. He demands Moo Chi to find out the reason why the boy had to be kidnapped.
An hour-long live broadcast goes on air in hopes of saving Han Kook. During the live broadcast, endless tip-offs and reports from former victims of the predator flood into the broadcast station.
While Han Kook is still missing and his life is under threat, the police haven't found any clues on the culprit. When Moo Chi gets back on his feet and starts investigating Bong Yi's grandmother's case.
After a year, Moo Chi is assigned to a new team that collects and organizes criminal evidence. Ba Reum starts to work for a small precinct. On his first day at work, he accidentally finds a dead body.
Ba Reum's instinctive intuition helps Moo Chi get closer to the bottom of the mystery behind the knots. While investigating the suspects, Moo Chi and Ba Reum spot a man who stands out from the rest.
Ba Reum finds himself acting a lot more violent than before and realizes that it's getting harder to control himself. He even gets the urge to kill someone. Reum is terrified that the Yo Han inside him might take him over completely.
Ba Reum keeps hearing Yo Han's voice in his head telling him to kill people. Bong Yi runs into Kang Duk Soo, which makes her anxious. On a rainy day, Kang Duk Soo starts making a move again.
Ba Reum keeps a close eye on Kang Duk Soo's murder investigation. Bong Yi on the other hand, thinks Moo Chi was the killer of Kang and keeps her mouth shut even when she's being speculated as a suspect.
The DNA result found on the weapon that was used to kill Kang Duk Soo confuses Moo Chi. Bong Yi joins Hong Ju's team and starts investigating the Suseong serial murder case.
Ko Nam, a bright and insightful kid finds out about a cat killer around his neighborhood. One day, he promises to share his personal investigation with Bong Yi but unexpectedly goes through an accident.
Ba Reum starts to remember everything and is filled with guilt. When Ba Reum sees Han Kook's mother desperately looking for her child even after a year, he decides to confess his crimes and clear Yo Han's name.
Ba Reum finally remembers what exactly happened to his family. He tries to set a trap to catch the mysterious OZ, and he finds out a secret about Dr. Daniel Lee in the process. Meanwhile, Moo Chi finds the car the murderer used.
Moo Chi finds out that Ba Reum was the killer and ferociously charges after him at the station. Moo Chi also suspects that Detective Lee is part of the OZ organization and tries to carry out an investigation on the two, but it's not easy.
Moo Chi and Bong Yi get closer to the truth. In the meantime, Ba Reum tries to find his aunt, the last missing puzzle piece. Sang, who tries to confirm Ba Reum's alibi, finds something suspicious at Ba Reum's house.
Ba Reum makes the final decision to turn himself in and finish one last mission that only he can do. Hong Ju also shares her big secret that's been tormenting her for years.
