Episode 10 of "Undercover High School" kicks off with a grandiose re-entrance by Seo Myeong-ju, accompanied by Principal Park, sending ripples through the school corridors. Hae-seong arrives just in time to witness this dramatic scene in the foyer, as Principal Park commands all teachers to retreat indoors. Myeong-ju, with a hint of triumph in her eyes, reveals her awareness of the cryptic clue involving the bead and the rising sun, subsequently instructing Hae-seong to vacate the premises. The chairwoman promptly expels Hae-seong, who, with a heavy heart, packs his belongings and bids farewell to his friends.
Gathering his team, Hae-seong and his compatriots surmise that Director Kim must have orchestrated Myeong-ju's release. Determined to persist with their investigation, they are abruptly interrupted by NIS internal investigators who stage a raid on their office. To their astonishment, the agents "stumble upon" a stash of illicit cash, leading to Seok-ho's arrest.
Myeong-ju, later, phones Hae-seong, her voice tinged with mischief, inquiring if he appreciated her parting gesture. She issues a stern warning to cease his inquiries and even threatens Soo-a's well-being. Later that evening, Soo-a finds Hae-seong drowning his sorrows at a quaint tent bar. In defiance of Myeong-ju's threats, Soo-a urges him to push forward at all costs, handing him the student council book marked with a photograph of Hae-seong's father.
That same night, Seok-ho reaches out to Hae-seong, relaying the disheartening news of their team's disintegration. Undeterred, Hae-seong expresses his resolve to continue and seeks his team's support, which they readily grant. Elsewhere, Myeong-ju confronts Director Kim, acknowledging his role in her release but far from grateful, she reminds him of her influence in elevating him to his current position.
Within the school walls, an unsettling transformation takes place as meals are now dispensed based on academic performance. Dong-min and his peers are relegated to dining at a later hour. Meanwhile, Yoo-jung updates Hae-seong over the phone, unaware that Ye-na, eavesdropping, catches snippets of their conversation.
Hae-seong and his team initiate their investigation at his abode, where he unveils the student council book harboring a mysterious fourth ghost tale. This narrative speaks of a computer spontaneously illuminating, displaying words penned by a deceased educator. However, considering the era in which Seo Byeong-mun authored these tales, computers were but a distant dream. Hae-seong deduces that his father crafted this fourth tale solely to perplex Myeong-ju, positing that the true collection only comprised three chilling stories.
Their attention now pivots on an enigmatic phrase: "When the one with the bead confronts the red sun, the gold shall glimmer." They surmise that the 'bead' alludes to Seo Byeong-mun, speculating that the statue of him gracing the school's premises might hold vital clues. Hae-seong postulates that the 'red sun' might symbolize Japan.
Disguised as a deliveryman, Hae-seong slips into the school's hallowed grounds. Soo-a, recognizing him instantly, tails him to the statue of Seo Byeong-mun. They discern that the statue is rotatable, but their exploration is abruptly halted by Vice Principal Baek's arrival. In a bid to aid Hae-seong's escape, Soo-a is compelled to divert Baek's attention.
Regrettably, Baek later rats out Soo-a's delivery escapades to Myeong-ju, who swiftly deduces Hae-seong's clandestine visit. She instructs Principal Park to send Hae-seong her second gift as a disciplinary measure. Meanwhile, Ye-na confronts her mother about her awareness of Hae-seong's fake student status. Myeong-ju dismisses it as none of Ye-na's concern and suggests a mother-daughter stroll to bond. As Myeong-ju exits, Ye-na catches a glimpse of something incriminating on her tablet.
The ensuing day, a storm brews at school as everyone receives pictures depicting Soo-a and Hae-seong together, falsely suggesting an improper teacher-student relationship. Soo-a heads to her class, only to be intercepted by Principal Park, who directs her to see Myeong-ju. Just then, Hae-seong strolls in with an air of confidence, brushing Park's hand off Soo-a. Boldly, he announces to the spellbound students that he is, in fact, 31 years old.
The news of Hae-seong's return to his classroom ripples through the school, drawing a crowd of curious students. He reveals to them that his return was driven by an unfulfilled goal: acquiring his high school diploma. Furthermore, he confessionally admits his affection for Soo-a. Just in time, Myeong-ju arrives to witness the students' jubilant welcome. As Hae-seong departs, he warns Myeong-ju to be cautious.
Myeong-ju is incensed by this unexpected twist, realizing that Ye-na must have tipped off Hae-seong, enabling his swift response. In the meantime, Hae-seong expresses gratitude to Ye-na for her assistance. She inquiries whether his return was prompted by her mother. He vows to unveil the truth to her at a later juncture. Later, Ye-na returns home to a bombshell dropped by her mother—plans to send her abroad for further studies.
Elsewhere, Hae-seong and his team deliberate on their next steps. Hae-seong asserts that they must locate the gold bars the same night to outpace Myeong-ju. Although he has a plan in mind, additional manpower is required. To everyone's astonishment, including Soo-a, her mother reveals herself as a retired member of the infamous Bloody Chicken Gang and recruits her former comrades to aid in their mission. Before embarking on their daring endeavor, Soo-a suggests a meal with Hae-seong post-mission.
Myeong-ju finally connects the dots with Seo Byeong-mun's arrest. She hastily makes her way to the school, where Soo-a's mother and her gang have constructed a formidable barricade. Introducing himself as a NIS agent, Hae-seong informs Myeong-ju of their investigation into a North Korean spy潜伏 within the school. Myeong-ju attempts to contact Director Kim, but Mi-jung cunningly intercepts her efforts.
While Myeong-ju seeks backup, Hae-seong ventures inside. He manipulates the bust, causing the entire pedestal to slide aside, uncovering a hidden trapdoor. In the cliffhanger ending of Undercover High School Episode 10, Hae-seong descends into a vast, desolate chamber—yet finds no gold bars. Instead, he discovers a skeleton adorned with a wristwatch, undeniably identifying it as Hae-seong's father.
