Episode 4 of The Scarecrow begins with Young-woo and Tae-joo engaging in a heartfelt conversation about their past, leading them to share intimate stories from their personal lives in order to deepen their understanding of each other. The episode then takes us back to 1988, where Tae-joo awakens in a hospital. He enters the autopsy room to witness the mutilated body of Min-ji, whose chest bears an inscription from the serial killer's knife. The doctor also reveals that the killer had stuffed Min-ji's genitals with sand and toffees, causing Tae-joo to recognize the toffee as the one Min-ji had given him during their final conversation. He is overcome with sadness and begs Si-young to let him rejoin the case in order to catch the killer. Si-young ponders this request, wondering if they can once again work together.
A flashback shows Si-young approaching Tae-joo in high school and inviting him to be friends. Tae-joo initially refuses, but Si-young offers to pay him for tutoring sessions. During these sessions, Si-young's mother leaves snacks for them, which Tae-joo secretly packs for Sun-young. One day, Si-young asks Tae-joo to attend the same college as him, but Tae-joo's mother tells him to earn the money himself. The next day at school, Tae-joo tries to approach Si-young, only to be pushed away by him. Si-young mocks Tae-joo's mother's profession and distances himself from him.
Back in 1988, Police Chief Park Eun-kyu discusses the case with the officers, where Man-chun criticizes Tae-joo's termination and claims that Kyung-ho lied about the assault. The Superintendent of Police, Cha Jun-young, calls Chief Park and requests his reinstatement of Tae-joo. Man-chun informs Tae-joo of this news, leaving him surprised.
Meanwhile, Si-young thanks Superintendent Cha for the favor, offering his help in his father's elections in exchange. He is mocked by others for trying to enter politics through marriage to Hee-jin, the daughter of politician Kim Tae-seop.
At the station, Dae-ho is upset with Tae-joo for leaving the case. Tae-joo takes him out for a meal to calm him down and explains that he quit to keep his promise to Min-ji, who had asked him to find the killer. Tae-joo declares that he will not rest until he catches the perpetrator.
As he leaves, Tae-jo sees the blurry photo of the killer published by Ji-won and wonders if it will lure out the suspect. At the same time, the killer sees the photo and panics. Tae-jo visits the newsroom to speak with Ji-won about the case, discussing how she had hidden her camera in the field while being chased. Tae-jo notices the handkerchief covering the killer's face in the photo and tries to recognize it.
Meanwhile, Sun-young takes a break from school to recover. Elsewhere, Si-young has a meal with Hee-jin when they run into her friend. The woman asks Si-young to introduce her to his single friend, but Hee-jin pulls him away. Si-young notices Hee-jin's reaction and asks about it. She explains that the woman's mother was a mistress, which is why she dislikes her. This unsettles Si-young, and he leaves.
While driving, he recalls his childhood and almost hits Sun-young but stops in time. He apologizes and takes her out for snacks after hearing her stomach growl. They talk about her break from school, and he brings her more snacks. He offers to drive her home but she refuses to get in the car with him.
As she walks away, Kyung-ho follows her and attempts to attack her. Gi-beom arrives and stops him. Kyung-ho insults Sun-young, and Gi-beom attacks him, choking him in anger. At the same time, Tae-joo goes through Min-ji’s art supplies and finds sketches of the suspect. He questions Ki-hwan about the night of her death. Ki-hwan reveals that Gi-beom followed Min-ji to give her an umbrella. Soon after, other officers arrive at the library to arrest Gi-beom for assaulting Kyung-ho. Tae-joo informs his team that Gi-beom is now on the run. He visits Si-young to discuss Gi-beom as a witness. He asks Si-young to convince Kyung-ho to drop the charges to prevent Gi-beom from fleeing. Si-young agrees but suggests Gi-beom could be a suspect.
Meanwhile, Un-hui and Dae-ho inform Ji-won and Sun-young about the incident. They are shocked to hear about the attempted murder charge. Ji-won consoles Sun-young and drops her off at home. Instead, Sun-young goes to the place where she is hiding Gi-beom. A flashback shows her stopping him from killing Kyung-ho while a passerby calls the police. She convinces Gi-beom not to turn himself in.
Later, Sun-young asks Gi-beom to run away with her. Meanwhile, Si-young visits Kyung-ho in the hospital. He pressures him to drop the charges. When Kyung-ho questions him, Si-young threatens to damage his and his uncle’s reputation. Kyung-ho agrees, but Tae-joo overhears the conversation. He asks Kyung-ho if Si-young exposed the truth about his mother’s profession. Kyung-ho confirms it, and Tae-joo recalls being bullied after Si-young exposed his mother’s profession.
Meanwhile, Ji-won notices that Sun-young’s handkerchief has the same embroidery as the one used by the killer. That night, Si-young drinks with Chief Park. On his way out, Tae-joo confronts him about targeting his sister. Si-young dismisses it as a misunderstanding, but Tae-joo recalls how he was humiliated in the past. Si-young says he wants to start fresh, hoping Tae-joo will forgive him. Tae-joo refuses, but Ji-won interrupts and shares her discovery about the handkerchief. She suggests Gi-beom could be the killer. Dae-ho arrives and recalls his earlier conversation with Sun-young. Tae-joo orders a search for Gi-beom and Sun-young.
At the same time, Gi-beom locks Sun-young in a room and escapes alone. Sun-young tries to get out, but the real killer reaches her first. Si-young arrives in the fields and searches for her. He finds her belongings and eventually finds her unconscious. He looks for the killer and is about to attack when Tae-joo arrives with Ji-won. They find Sun-young’s belongings in Si-young’s car and grow concerned. A gunshot is heard, prompting Tae-joo to run into the field, where he finds Si-young carrying Sun-young.
Si-young says the killer escaped and that he was shot during the struggle. As Tae-joo calls for help, Si-young speaks about their past. A flashback shows Si-young overhearing Tae-joo’s mother refusing to pay for his education. He decides to make Tae-joo hate him so he will not follow him to college. Later, Si-young sees his father with Tae-joo’s mother. Back in 1988, Si-young tells Tae-joo they are half-brothers before losing consciousness. Meanwhile, Gi-beom is arrested by Hyung-gu, while the real killer continues searching for another victim. Tae-joo rushes Si-young into surgery.
In 2019, Young-woo asks if Tae-joo still keeps in touch with Si Young and whether working with him changed his life. The episode comes to an end with Tae Joo revealing that he lost everything because he teamed up with Si Young
