Episode 8 of "Undercover High School" kicks off with an intriguing twist as Hae-seong stumbles upon the meticulously planned blueprint for the 'Undercover High School' mission on Myeong-ju's computer screen. With a sense of urgency, he and Soo-a depart, Hae-seong heading back to his team's base while deep in thought. Their conversation zeroes in on the enigmatic 'Do' key on the piano, hinting that silence itself might hold the next clue. Seok-ho perceives a subtle shift in Hae-seong's demeanor but brushes it off as nothing significant.
Soo-a lingers behind, lending a helping hand to fellow educators in tidying up the school premises. Her eyes catch a distressing scene: Han Seung-jae, a student, berating a server for accidentally spilling food on his shoes, an encounter made worse by the arrival of Seung-jae's father, neither of whom bother to apologize. As they exit, the server, trembling slightly, hands Soo-a a mysterious black card that had inadvertently slipped from Seung-jae's pocket. Meanwhile, Myeong-ju ponders thoughtfully by her piano, contemplating the successful execution of the third clue.
Elsewhere in the narrative, Young-hun attempts to renew his advances towards Mi-jung, only to be gently rebuffed with the revelation that she is seeing someone else. Transitioning to the domestic sphere, Soo-a meticulously deciphers the 'Do' notes from the school anthem—specifically, those played by the deceased girl featured in their narrative—piecing together the next cryptic hint: "When the one with the bead faces the sun, the gold will shine."
She tracks down Hae-seong in a bustling grocery store, presenting him with this latest enigma. His response, however, is distant and detached. Frustrated, Soo-a insists he accompany her for a drink, where he finally unburdens himself about his discovery in Myeong-ju's office and his trust issues within the NIS. Soo-a, empathetic, offers her unwavering support.
As they stroll back home under the fading light, Hae-seong drops a bombshell: he is Si-hyun, the little boy from Soo-a's past whose abrupt disappearance from school in second grade left her puzzled and heartbroken. The revelation sends shockwaves through Soo-a, prompting a poignant conversation about the reasons behind their name changes. She recalls how his absence, triggered by his father's mysterious disappearance, left an indelible mark and offers words of comfort and renewed hope.
The following day at school, Ri-an urges Soo-a to delve into the website mentioned on the enigmatic black card, fearing it might harbor some hidden malice. Obediently, Soo-a accesses it and stumbles upon a meticulously compiled list detailing students' grades across semesters.
Meanwhile, in class, Ye-na’s thoughts drift back to the sight of Seung-jae's father handing him the mysterious card. Curiosity piqued, she tracks him down to the council room where he’s frantically searching for it. Confronting him, she inquiries about how his father could have foreseen his victory in an essay contest that hadn't even taken place yet. Seung-jae, reluctantly sharing the card's secrets, reveals a startling truth: Ye-na's consistent top rankings were actually due to Myeong-ju’s manipulations.
Horrified, Soo-a hastily closes the website but her actions draw unwanted attention, leading to Myeong-ju being informed of her breach. Catching Soo-a fleeing from the staff room, Myeong-ju issues her a chilling warning about adhering to the established rules.
Panicked, Soo-a rushes to Hae-seong for help, but he brushes her off, citing his current overwhelming responsibilities. However, his resolve begins to waver when he encounters Yoo-jung diligently studying in the library, her aspirations to become an NIS agent like him shining brightly in her eyes. This encounter seems to ignite a change of heart in Hae-seong, making him reconsider his stance on assisting Soo-a with her predicament.
That night, Soo-a persists in her research endeavors, but encounters a roadblock when the URL mysteriously ceases to function. Elsewhere, in a bustling restaurant, Myeong-ju confronts Seung-jae and his father, revealing that she has deleted the URL. Seung-jae drops a bombshell, informing Myeong-ju that Ye-na is also privy to the information, leaving her staggered. He labels Ye-na as obnoxious, prompting his father to urge him to leave the premises. Meanwhile, Hae-seong eavesdrops on their conversation through a concealed listening device.
Myeong-ju, vengeful, mentions a plan to teach Soo-a a harsh lesson, which spurs Hae-seong into immediate action, rushing to Soo-a's side. In a secluded corner of the restaurant, Myeong-ju catches up with Seung-jae in the men's room and physically confronts him until he utters an apology. Meanwhile, Hae-seong locates Soo-a at the school, engrossed in unraveling the grades scandal. She recounts a harrowing tale of a former student falsely accused of stealing an exam paper, a situation she couldn't adequately address as a temporary teacher. Now, with renewed determination, she vows to shield her current students from injustice. Hae-seong, aware of Myeong-ju's knowledge, steps in to offer his assistance.
Together, they transport all of Soo-a's documents to Hae-seong's place and embark on a thorough investigation. They compile a list of parents colluding with Myeong-ju and trace their connections to her Edu-city project. Hae-seong deduces that the NIS agent is likely to benefit from the Edu-city project as well. Armed with this intelligence, Hae-seong storms into the NIS headquarters and updates Mi-jung and Young-hoon on the unfolding drama. Seok-ho enters the scene, ordering Hae-seong to concentrate solely on locating the gold bars and steer clear of school-related matters. Director Kim, catching them in a heated discussion, summons them to his office, where Hae-seong briefs him on the situation. The director ponders on the matter. Shortly after, Myeong-ju receives a call, alerting her to Hae-seong's involvement.
Later in class, Hae-seong receives a text message informing him that his mission has been abruptly terminated. He shows the message to Soo-a and declares that he must now lure Myeong-ju into a trap. Pretending to possess knowledge about the location of the gold bars, Hae-seong intends to secure his position at the school. Fortunately, an unexpected opportunity arises when Myeong-ju decides to have lunch at the school on that very day. Hae-seong swiftly heads to the cafeteria and manages to secure a seat next to her. Casually, he inquiries if the gym has undergone any renovations, prompting her to respond that it hasn't and to wonder if he's referring to the infamous fourth ghost story.Hastily finishing his meal, Hae-seong excuses himself and leaves.
That evening, when Myeong-ju returns home, she finds Ye-na awaiting her presence. Confessing her involvement in manipulating the grades, Myeong-ju even goes so far as to disparage Yoo-jung. Ye-na, loyal to her friend, reprimands her mother for speaking ill of Yoo-jung.
A nostalgic flashback transports us to a bus ride where Ye-na and Yoo-jung first became friends. Ye-na, discovering that Yoo-jung had secured the second-highest position in her class without any private tutoring, decided to forge a bond with her. Presently, Ye-na visits Yoo-jung's house, where Yoo-jung's mother prepares dinner for both girls. Grateful for the meal, Ye-na bids farewell to Yoo-jung and heads back home.
Meanwhile, Soo-a's investigation uncovers a potential link between her colleague Lee Jun-ho and the grades scandal. Taking her belongings to Hae-seong's house, she finds the main gate unlocked. Venturing inside, she encounters a shadowy figure dressed in black. Just then, Hae-seong arrives home to find Soo-a lying unconscious within his residence.
A poignant flashback concludes Undercover High School Episode 8, showing Soo-a in conversation with Min-ji, a former student from her previous school. With earnest sincerity, Min-ji urges Soo-a to fulfill her promise and become a full-time teacher one day.
