<Korean TV Series NOW> ”Undercover High School” EP6, Seo Kang Joon reveals his true identity to Jin Ki Joo = Viewership rating 6.7%, Synopsis/Spoiler

*This article contains spoilers. In EP6 of the MBC TV series "Undercover High School" (viewership rating: 6.7%), Hye-sung (Seo Kang-Joon) reveals her true identity.
Hee-sung and Soo-ah (Jin Ki Joo) begin working together. A security guard named Kim (Koo Min-hyuk), who was supposed to meet with Hye-sung, falls from the rooftop to his death after a fight with a mysterious person.
Hye-sung was supposed to meet Kim, but as soon as she arrived at the school, she found him collapsed and checked the notebook of her missing father, Jaehyun (Oh Eui-sik), in his hands.
At this time, Hye-sung chased after the man who had been watching her and headed to the roof. At the moment she was fighting with the mysterious man, she heard a noise from outside the rooftop door, and while Hye-sung was frightened and turned around, the suspicious person fled.
It turns out that Myung-ju (Kim Shin-Rock) and Jae-moon (Park Jin-woo) were behind the whole crime. After the situation calms down, Hye-sung takes the notebook and goes to the police station to find Director Kim (Lee
He grabs Director Kim by the collar, questioning why Mr. Kim, the security guard, had the notebook, and Hye-sung realizes that her father's final mission was the same as her own.
Hye-sung was also angry at the situation of her father who went missing after completing her mission and the fact that all of this was hidden. Meanwhile, Hye-sung is trying to uncover the secret hidden in the clock by joining the student council.
He opened the clock and matched the keywords written in the notebook, and the Byeongmoon High School school song began to play from inside the clock. Hye-sung, who had recorded the clock's sound, was about to leave the student council room.
Just then, Su-ah appears. Su-ah claims that she saw Hye-sung on the rooftop on the day of Kim's death and is suspicious of his identity. Hye-sung tells Su-ah that she is an agent of the National Intelligence Service and her purpose in infiltrating the building is to
revealed all of the above.