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The nation is thrown into a frenzy as a string of murders occur. These become known as the "Headhunter" murders as the victims are beheaded. Moo-chi, the only survivor of the incident, provides a decisive clue as to who the Headhunter is.

Moo-chi visits the Headhunter, who is on death row and awaiting execution in a cell at Moojin Detention Center. Moo-chi’s only goal in life is to avenge his family’s death and kill the Headhunter himself. Another monster grabs Moo-chi's attention.

Chi-guk is found in the detention center with multiple stab wounds. Before the shock of the incident subsides, Chi-guk's watch, a gift from Ba-reum and Dong-gu, is found at the grape farm murder scene. Ba-reum starts investigating with Moo-chi.

The Predator kidnaps their next prey, and the nation falls into panic. Moo-chi gets a call from the killer demanding he find out what the victims have in common before he kills again. Ba-reum and Moo-chi struggle to find the answer.

Only an hour is left to save Han-guk’s life. While everyone struggles to figure out the Predator's motivation, Ba-reum secretly heads somewhere. During the live broadcast, reports on the victims and witness testimonies about the Predator pour in.

Moo-chi loses his mind after seeing Mu-won found in a terrifying state. Giving up on everything, he shuts everyone out. Meanwhile, the chances of the child's survival decrease drastically, and things aren't looking good for Moo-chi.

Moo-chi is assigned to the Evidence Team led by Du-seok. Meanwhile, Yo-han is shot by Moo-chi and is taken to the hospital. Ba-reum looks like a completely different person when he wakes up after his surgery with lifeless eyes.

The investigators discover that the signature knot of the knot murder case is identical to the knot found with the remains of Du-seok’s daughter. Meanwhile, Ba-reum puts his instincts to use as he starts a joint investigation with Moo-chi.

Through Han Seo-jun, Ba-reum learns the shocking truth of the accident one year ago and suffers from horrible memories that keep popping into his mind. Eventually, an uncontrollable instinct to kill takes over him.

Ba-reum trembles in fear as Yo-han slowly takes over him. A copycat murder of the Moojin serial murder occurs, and Ba-reum searches for the culprit more desperately than anyone else in the hopes of proving he is not a psychopath.

“Kill! Kill!” Whispers of Yo-han echo in Ba-reum’s mind. Ba-reum eventually kills Wu Hyeong-cheol and then decides to turn himself in. However, as soon as he makes that decision, someone appears in front of him and makes a shocking proposal.

Ignoring Daniel's advice, Ba-reum basks in the thrill of the instinct to murder, an instinct that has finally awakened. Whenever he follows his instinct, he tries to cover his tracks. But then, some decisive evidence is found.

Moo-chi is shocked by the results of the test done on the culprit's DNA that was collected from the weapon the culprit used to kill Deok-su. While looking into the case, Bong-yi also discovers a shocking truth about the day of the incident.

Ba-reum crosses paths with Moo-chi at the scene of the incident. The chase between them begins. Bong-yi learns that the true culprit behind the murder of Deok-su is Ba-reum. She waits for Ba-reum in front of his house, only for him to push her away.

Ba-reum’s memories suddenly come flooding back. About to go insane from guilt, Ba-reum eventually finds himself in front of Chi-guk, who knows about his crime. Meanwhile, Moo-chi rushes to the hospital, only to learn a shocking truth.

Moo-chi is confused by Ba-reum’s suspicious behavior. Ba-reum, thinking back on his last meeting with Chi-guk, realizes there's a strange figure at large. While figuring things out, he learns shocking truths about himself.

Ba-reum finally recalls the entire truth of the Guryeong family murder case. He sets up traps to catch OZ and reveal their identity. Meanwhile, Moo-chi finds OZ's fake vehicle and is left shocked to see Ba-reum in the dashcam footage.

Ba-reum puts the final pieces of the puzzle together. Moo-chi confronts him about his suspicions. Meanwhile, Bong-yi starts to suspect Yo-han isn’t the real culprit and runs to the Moojin Police Agency, where she learns a shocking truth.

Moo-chi and Bong-yi get closer to finding out the truth, and Ba-reum seeks out the last person needed to solve the puzzle of OZ. While trying to confirm Ba-reum’s alibi, Detective Shin finds an unexpected item in Ba-reum’s home.

After everything's revealed, the police arrest Choi Yeong-sin but are forced to release her due to a lack of evidence. Amid the chaos, the presidential election finally begins.