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Undercover High School, starring Seo Kang-joon and Jin Ki-joo aired episodes 3 & 4 on MBC on February 28 and March 1, 2025. Undercover High School tells the story of an NIS agent, Jeong Hae-seong, depicted by Seo Kang-joon, who goes undercover as a high school student for a secret mission.
Jeong Hae-seong grabs everyone's attention at school with his good looks while looking for King Gojong's gold worth 800 million KRW, which seems to be hidden somewhere in the school.
Meanwhile, Jin Ki-joo plays the role of Oh Su-ah, Jeong Hae-seong's homeroom teacher. Oh Su-ah teaches Korean history and is employed on a contract basis, working hard to get promoted to a full-time position. At some point, Oh Su-ah believes that Jeong Hae-seong, the newly transferred student, looks similar to her first love.
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In episode 4 of Undercover High School, Seo Kang-joon's character plans to get into the student council as a part of his mission to find the second clock. However, he needs to get 100 students' signatures as approval. Despite his looks, he does not have any friends besides Lee Dong-min. Furthermore, another condition was to get a place in the top ten scorers in midterm exams.
His NIS team leader, Ahn Seok-ho, told him to focus on studying while they acquired signatures. Along with his colleagues Park Mi-jeong and Go Young-hoon, Ahn Seok-ho prepared a show with free food at their cafe to attract students.
As Jeong Hae-seong reached the café, he was embarrassed and tried running away. Ahn Seok-ho caught him and sent him up on stage. Jeong Hae-seong had no option but to dance and sing along like an idol to get the signatures.
Undercover High School ep 3-4 recap: Seo Kang-joon’s Jeong Hae-seong becomes a fellow classmate Lee Dong-min’s savior
In Undercover High School episode 3, Jeong Hae-seong, played by Seo Kang-joon, learns that the skeleton in the old building was fake. He found a seal near the skeleton that resembles King Gojong's stamp.
Meanwhile, school director Seo Myung-joo, who had her eyes on Jeong Hae-seong entering the old building, asked the principal to let him continue doing what he wanted. As his actions may lead her to the gold.
Jeong Hae-seong convinced his NIS team that the seal and the urban legends at school may be connected to the gold and that he would continue looking into other stories as well.
On the other hand, in Undercover High School episode 3, Oh Su-ah gets furious and accuses Jeong Hae-seong of scamming Lee Dong-min's grandmother for her ID. Lee Dong-min heard her scolding Jeong Hae-seong but ignored them, knowing it was Park Tae-su who took the ID. Oh Su-ah witnessed Park Tae-su bullying Lee Dong-min and confronted him, but both denied the same.
Later in the Undercover High School episode 3, Oh Su-ah bumps into Jeong Hae-seong at a convenience store and apologizes to him for misunderstanding. Soon, Lee Dong-min's grandmother was arrested in the case of allegedly running an illegal gambling website. The next day, Oh Su-ah posts an anonymous comment on the school bulletin about Park Tae-su bullying Lee Dong-min.
Due to this, Seo Myung-joo held a meeting regarding this with Park Tae-su’s parents. Lee Dong-min once again denied being bullied by Park Tae-su and blamed it on Jeong Hae-seong. Notably, Park Tae-su blackmails him. This led the school committee to hold a meeting to expel Jeong Hae-seong. By the end of Undercover High School episode 3, Jeong Hae-seong brings in a witness against Park Tae-su at the meeting.
In Undercover High School episode 4, as the witness who was the real owner of the illegal gambling site testified against Park Tae-su, Lee Dong-min also spoke up, and Park Tae-su was expelled. Jeong Hae-seong later learned from Lee Dong-min that a girl pushed her classmate from the top floor as she was jealous of her topping the class as she came second every time.
Soon, at midnight, the clock struck, and the girl who died returned with red eyes, and the student who killed her was found dead in the washroom. This is why the students are warned not to stay in school at night.
Jeong Hae-seong learned that there were two clocks related to the urban legend, and one was inside the student council room, where no one except members of the council were allowed. He decided to get a place on the student council squad.
Meanwhile, Jeong Hae-seong’s younger sister Ahn Yoo-jung, who was on the council, learned that Lee Ye-na was interested in Jeong Hae-seong. Ahn Yoo-jung is seemingly mistreated by Lee Ye-na, who was acting as a friend but did not see her on the same level as she was not rich.
When Lee Ye-na learns Ahn Yoo-jung is against Jeong Hae-seong joining the student council, she flips on her. Ahn Yoo-jung called Jeong Hae-seong on the rooftop of the school and warned him not to join the council as it may ruin her friendship with Lee Ye-na. She threatened him that she would blow his cover if he joined the team.
On the other hand, school director Seo Myung-joo was revealed to be Lee Ye-na's mother. Seo Myung-joo pressured Lee Ye-na to come first in class and take medicines prescribed to patients with ADHD symptoms to stay focused. As Undercover High School episode 4 moved to the end, Jeong Hae-seong missed school due to his training and informed Oh Su-ah that he was sick.
Later that night, Oh Su-ah told him that she was bringing him porridge as he told her he was too sick. However, he got in trouble as the rooftop house he was supposed to stay at had not been set up by his teammate. Ahn Seok-ho, Park Mi-jeong, and Go Young-hoon rush to set up the house. As Oh Su-ah approaches, the three hide in the closet.
Jeong Hae-seong tried to send her back as soon as possible, but the porridge fell on her pants, and she had to use his bathroom. He took this chance to send his teammates home, but Oh Su-ah came out earlier than expected. In the chaos, Oh Su-ah and Jeong Hae-seong fell down and locked lips. They got up swiftly, and his teammates also fell out of the closet.
Meanwhile, Undercover High School airs every Friday and Saturday on MBC at 9:50 pm KST.
