MADDER: I Am the Culprit in That Case

Yohara (Ioki Mao) reveals that she used Akane's (Mizuno Kyoshin) notebook to relive Akane's crime. Raised by a single mother, Yohara focused on studying from elementary school onward, aiming to get into Seiran Academy. However, before she knew it, she began to look down on people with lower academic ability than her, belittling her own family background and pushing her mother away from school. Her mother was deeply disappointed by the change in her daughter. They got into an argument, and Yohara pushed her mother down the stairs, killing her. The next morning, her mother's body was found in the park with her left arm severed. Why did Kurokawa take on the crime in Yohara's place and commit the heinous act of murdering Shinozaki (Yoshina Riru)? And what became of Akane, who was captivated by Kurokawa and turned into a serial killer?!

A woman claiming to be the culprit visits detective Kajitani (Takeda Rina) from the investigation headquarters. Kajitani, who is pursuing a serial killer who has shocked the whole of Japan, warns her by saying, "If it's just a prank, I don't have time to bother with it," but the woman says, "I don't think there will be any more victims," ​​and hands her a notebook. After reading the notebook, Kajitani begins to talk as if looking back on the serial murders that occurred at Seiran Gakuen and its surrounding areas six years ago. The woman then reveals that there was a student who was fascinated by the suspect Kurokawa (Yamamura Ryuta), and that the person began committing small crimes at the school before eventually becoming a serial killer.

In 2002, 23-year-old Kurokawa (Yamamura Ryuta) was working as a secretary apprentice for his father, Toshihisa (Kato Daiki), a politician. Tired of the world of pretense and political motives, he finds himself at the only place he can find himself, the cafe "Rindon," where he grows closer to the cafe manager's niece, Ryoko (Sato Miyuki). A year later, when Toshihisa finds out about the two's relationship, he investigates Ryoko's background and firmly opposes her, saying she is not suitable for his son. Furthermore, when he finds out that Ryoko is pregnant with Kurokawa's child, he takes it upon himself to help her. Learning of Kurokawa's position, Ryoko disappears, saying, "I will raise this child."

Akane (Ioki Mao), who participated in the Quiz Research Club's friendly tournament held in Kurokawa's (Yamamura Ryuta) hometown, sneaks out of the venue and visits a coffee shop that has a connection to Kurokawa. However, when she mentions Kurokawa's name, the manager's face changes color and she is quickly kicked out. Then she sees Eto (Higuchi Kohei) chasing Akane outside the store. Akane asks Eto for a favor and sneaks into the back of the store without the manager noticing. She sees a letter and a photo of a woman, Kurokawa, and the manager looking intimate with each other. Eto, who witnessed Akane's bold actions, confronts Akane with a suspicion he has had for a long time. "You're the culprit, aren't you?" Then, Eto takes an unexpected action...

Kurokawa (Yamamura Ryuta) turns himself in. Akane (Ioki Mao), unable to understand why Kurokawa, who she thought had fled, turned himself in so easily, is overcome with a sense of emptiness, in contrast to the relief and excitement around her at the arrest of the culprit. Meanwhile, Akane visits the Shinozaki family with Yohara (Mizuno Kyoshin) at the request of Shinozaki's (Yoshina Riru) father, Hirohiro (Kagaya Kei), who wants to talk to his daughter's classmates. However, while Hirohiro is crying over his daughter's sudden death, he also seems to be trying to extract some information from Akane and the others. Noticing this, Akane gets close to him, pretending to regret the death of her classmate, and asks to see Shinozaki's room. Then, she discovers some surprising data on Shinozaki's tablet...

The body of a classmate without a left arm was found on the rooftop. As the students panicked, Akane (Ioki Mao) found Kurokawa (Yamamura Ryuta) with a cold expression on his face on the other side of the crowd. In response to the incident, the students were forced to go home, but Akane went to the rooftop after the scene investigation was completed. Then, Eto (Higuchi Kohei) and others arrived and accused Akane of being the culprit. The motive was that the victim, who had recently become interested in Akane and was sniffing around her, had become a nuisance. However, Akane burst out laughing at such a naive idea. However, it seems that Kajitani (Takeda Rina) of the First Investigation Division also considered Akane to be one of the possible suspects, and Akane was questioned.

The Seiran Unification is to be carried out on a new date, and Akane (Ioki Mao), who wants to believe in people's conscience, slips a card into the belongings of the foolish geniuses. Meanwhile, Kurokawa (Yamamura Ryuta), who was cleaning up the principal's office, discovers a wiretap. Kurokawa instantly realizes the identity of the culprit and says something meaningful to Akane, who is enjoying a crime-like act. His words make Akane even more interested in Kurokawa.One day, Shinozaki (Yoshina Riru) brings up the story of a murderer who is making waves in the world, and provokes Eto (Higuchi Kohei) and the others, speaking as if she knows his identity.

The Unification of Seiran is suddenly canceled! Behind it all lies the motive of an analogue teacher who is fighting alone in rebellion against the school! Akane (Ioki Mao) motivates the foolish geniuses to move on to the next plan...

The culprit himself is working behind the scenes to solve the stone statue destruction case! With the assistance of Akane (Ioki Mao), who has grown impatient with the lack of progress in the search for the culprit, the truth is revealed! What is the secret hidden in the stone statue?

Nakano Akane (Mao Ioki), a genius high school student with a standard deviation of over 78, meets Kurokawa Yu (Ryuta Yamamura), a young man who is believed to be the perpetrator of a murder case, and causes a small incident at school, which eventually leads to a serial murder case that will shock Japan.