Shin Ha Ru, a prominent neuroscientist, is in a pickle when the rumor of his forged brain donation consent form spreads. Things get out of hand when his current and previous unethical work practices garner attention and criticism from the media. Myung Se turns out to be the one who first spread the rumor, and it’s only a matter of time before Ha Ru finds out about it.

A famous guitarist dies while performing live on a stage, and the case is assigned to Neuroscientific Investigation Team as its first assignment. Like it or not, Myung Se is stuck with Ha Ru, and they are to solve the case together. It is the guitarist’s wife who confesses that she’s the murderer. Ha Ru right away recognizes that she suffers from Wernicke-Korsakoff syndrome, and things take an unexpected turn.

A body of a female Vietnamese victim whose face and hands are burned is found, and the prime suspect is her husband, who suffers from prosopagnosia, also known as face blindness. On top of that, even the victim’s best friend has disappeared. Ha Ru and Myung Se look into the case, and while doing so, they get into a sticky situation.

With new evidence and a new suspect, Ha Ru and the members of Neuroscientific Investigation Team close in on the culprit with the help of a brainwave test. Meanwhile, after going through the accident, Captain Seul turns into a person with a totally different personality. To make matters worse, Hwang Dong Woo’s efforts to reach Ha Ru continue.

A six-year-old girl, Ji Yul, is in the interrogation room with Myung Se to be questioned for her father's murder case. Her mother claims that Ji Yul is the one who murdered her own father as she is possessed with a wicked spirit, and she must proceed with the salvation ritual to be saved by the Messenger.

Ha Ru manages to sneak out of the building with Ji Yul and Eun Jung. He promises Eun Jung to show her where the real wicked spirit that has been hurting Ji Yul is. Meanwhile, Ha Ru finds a suspicious video clip of Myung Se on his tablet PC, and he is thrilled that he finally has his weakness in his hands.

Ha Ru is certain that he finally put Myung Se into a corner, but Myung Se finds a way out of the situation thanks to his past good deeds. Ha Ru is sick and tired of Myung Se’s good deeds and wants to prove that Myung Se is a person with a two-faced nature. Myung Se, in the meantime, is working on a case that involves a man suffering from Capgras syndrome.

The same man who is suffering from Capgras syndrome stands in front of Myung Se and Ha Ru, confessing that he has just committed murder. At the same time, his wife begins to show symptoms of conversion disorder. One of the two must be the murderer, but pinpointing the killer doesn’t seem so easy. Later, Ha Ru announces that he will be leaving the Neuroscientific Investigation Team.

A kid, who is referred to as Reset Man, makes a call to the police and confesses that he has killed his mother. The Neuroscientific Investigation Team believes that he is suffering from anterograde amnesia and that his memories are reset every 30 minutes. Ha Ru rejoins the team when he hears about the name of the suspect’s disease. No clear evidence supports the theory that the suspect is guilty of the murder, and the investigation continues.

With help from the witness, Reset Man’s stepbrother is pointed out as the new suspect. A heart-rending, tragic story behind the two brothers is finally revealed, and the investigation progresses. Meanwhile, Myung Se and So Jeong are out on their first date. Right when the date is going so well, a desperate phone call from Mo Ran spoils the mood.

A note at the crime scene of the murder reads, "I did what my brain told me to." Later, the murderer turns himself in as he confesses his crime. The public is stunned by his identity and again by the fact that he’s requesting a reduced sentence. Without knowing whether it’s Hwang Dong Woo’s scheme to put Ha Ru on the witness stand, and with all eyes on him at the trial, Ha Ru begins answering questions.

While the public is focused on Ha Ru’s recent revelation regarding his brain, Ha Ru helps Mo Ran get to the bottom of her high libido. Surprisingly, Ha Ru’s assumptions are right, and Mo Ran turns out to be a victim of the wrong medication. Now that she knows it wasn't her fault that she and Myung Se went their separate ways, she wishes she could turn back time and go back to how things used to be.

After a breakup with Captain Seul, Myung Se spends an evening drinking with Ha Ru, which ultimately leads to Ha Ru accompanying him to his home. As Yi Na steps out of her room, Myung Se and Ha Ru come across a male student lying on the floor with blood on his head. An investigation into Yi Na’s case shows that she could be the prime suspect. Seeing how Myung Se is not in a sound state of mind, Ha Ru steps in to help.

Ha Ru decides to put his knowledge to use to prove Yi Na’s innocence, and he does it so fluently. This brings Ha Ru and Myung Se’s family a bit closer, at least that’s what Myung Se believes. However, Ha Ru’s sole purpose of making Myung Se go through what he had to go through remains. At the same time, he stumbles upon crucial evidence that Myung Se has done something illegal.

Ha Ru found out that his parents were not victims of a car accident but of a murder. To get to the bottom of the truth behind the murder, Ha Ru decides to do as requested by Hwang Dong Woo. Ha Ru is well aware of what Dr. Hwang wants to accomplish through him, but he takes the risk anyway. One wrong step can turn Ha Ru into a murderer, and he knows it more than anyone else.