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Undercover High School starring Seo Kang-joon and Jin Ki-joo aired episodes 9 and 10 on March 21 & 22, 2025, on MBC. Undercover High School is a story about a NIS agent, Jeong Hae-seong (Seo Kang-joon), who infiltrates a school to find gold bars worth 800 million KRW for a mission.
Meanwhile, Jin Ki-joo plays the role of Jeong Hae-seong's homeroom teacher, Oh Su-ah, who is always suspicious of Jeong Hae-seong. However, at times she also believes that he resembles her elementary school.
In Undercover High School episode 10, school director Seo Myeong-joo posted photos of Oh Su-ah and Jeong Hae-seong from a restaurant bar having drinks. This was to tarnish Oh Su-ah's reputation among students and staff. However, Lee Ye-na saw the photos on Seo Myeong-joo's tablet. She contacted Jeong Hae-seong and informed him about the same.
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As soon as the first photos started circulating, Jeong Hae-seong came to the school and explained to batchmates that he was not a minor. It is shown that he is about to reveal his job as a NIS agent.
But he saw Oh Su-ah and Ahn Yu-jeong signaling him not to, so he told everyone that he could not finish high school studies, so he decided to enroll and get a diploma now. He also confessed that Oh Su-ah was someone he liked a lot.
Undercover High School ep 9-10 recap: Jeong Hae-seong finds out the whereabouts of his missing father
In Undercover High School episode 9, Jeong Hae-seong took Oh Su-ah to the hospital as she was stabbed by an intruder at his place. He soon learned that the intruder was math teacher Lee Joon-ho, who was ordered by Seo Myeong-joo to bring Jeong Hae-seong's notebook.
Later at school, Lee Joon-ho gave Seo Myeong-joo a final notice saying he would not tamper with grades of students or do shady activities for her.
As the story progressed, principal Park Jae-moon tried to get rid of Lee Joon-ho. NIS teammates Go Young-hoon and Park Mi-jeong tailed Park Jae-moon. They saw him put Lee Joon-ho in danger and tried to meddle in but got caught themselves. Soon Jeong Hae-seong arrived and saved them. Jeong Hae-seong sent Lee Joon-ho to special quarters.
The next day Lee Joon-ho came clean, and Seo Myeong-joo was soon arrested by the police due to her interference in grade manipulation. It was revealed that NIS director Kim Hyung-bae was the mole, but he also did not save Seo Myeong-joo. By the end of Undercover High School episode 9, the school staff and students are prepared for a special inquiry from the education.
Seo Myeong-joo left everyone in shock with her surprising return to the school. In Undercover High School episode 10, Seo Myeong-joo expelled Jeong Hae-seong and Oh Su-ah could not stop her. Meanwhile, there was a sudden search and seizure at the NIS department; Jeong Hae-seong and Ahn Seok-ho's team was dismantled.
However, Jeong Hae-seong suggested that the new base would be his house, and the team regrouped to continue the mission. Jeong Hae-seong learned that the fourth and last legend nobody knew about was because it was created by his father, Jeong Jae-hyun who went missing as a teacher, Kim Jae-yeong, 20 years ago while he was also on the same mission.
As they solved the riddle, the clues led them to the bust statue. Later, Jeong Hae-seong and Oh Su-ah tried to look into the statue, and it opened, but they were interrupted by the vice principal and had to abort the task. Later, Seo Myeong-joo tried to create a scene at school through Oh Su-ah and Jeong Hae-seong's photos but failed.
The team held a meeting at Oh Su-ah's mother, Im Chung-ha's restaurant, and discussed how they would get to the statue bust with the tight security and no backup from the NIS. Im Chung-ha heard their conversations and decided to help them. It was revealed that Im Chung-ha ran a group of gangsters before starting a restaurant. Oh, Su-ah was unaware of her past. Im Chung-ha asked her former goons to back them.
Towards the end of Undercover High School episode 10, Jeong Hae-seong and his team went to the school at night. They were stopped by Seo Myeong-joo. Jeong Hae-seong used his NIS agent card, saying they were here to look for a spy from North Korea.
Seo Myeong-joo didn't believe him and called Director Kim Hyung-bae. However, it was shown that Kim Hyung-bae was playing golf at that time, and Park Mi-jeong took this chance to use his phone and distract Seo Myeong-joo.
Jeong Hae-seong was able to open the bust statue and go underground. He learned that there was no gold but saw a man's skeleton who wore his father's watch. The Undercover High School episode 10 concluded with Jeong Hae-seong bursting into tears. He realized his father had been trapped underground all along.
Meanwhile, Undercover High School starring Seo Kang-joo and Jin Ki-joo airs every Friday and Saturday on MBC.
