Episode 6 of "Undercover High School" unfolds with an intriguing scene: the school's security guard, clutching Hae-seong's notebook, stands vigilantly on the rooftop, awaiting an unknown visitor. But the person who emerges, cloaked in mystery and masked, has ulterior motives—to seize the notebook. A fierce struggle ensues, culminating in the guard's unfortunate plunge, leaving Hae-seong to stumble upon the guard's lifeless form and the notebook. Peering over the edge, Hae-seong catches a glimpse of the masked intruder and dashes to the rooftop, where a harrowing confrontation ensues. Amidst the scrap, a sudden noise diverts Hae-seong's attention, enabling the assailant to slip away. To Hae-seong's astonishment, the masked man unmasks as Principal Park, who hastily escapes in a waiting car alongside Myeong-ju.
Enraged and seeking answers, Hae-seong bursts into Director Kim's office, demanding clarification on why his father, Jeong Jae-hyeon's, notebook was discovered within school premises. Director Kim concedes that Jae-hyeon's final assignment mirrored Hae-seong's current mission. Just as Hae-seong lunges towards Director Kim, Seok-ho intervenes, preventing a violent altercation. Hae-seong is crushed to realize that the NIS had always been privy to his father's whereabouts. He conveys to Seok-ho his need for a respite and subsequently abstains from attending school.
Secluded at home, Hae-seong delves into his father's notes, uncovering harrowing tales that mirror his own discoveries. The next morning, the school is shrouded in sorrow and disbelief as news of security guard Kim Hyeon-ho's death spreads. The administration brands it a suicide, producing a suicide note supposedly found in his office. During the student council meeting, Ye-na confronts Yoo-jung, querying her feelings for Hae-seong. When Yoo-jung unequivocally denies any affection, Ye-na vows to pursue him herself.
Meanwhile, back at the NIS, Director Kim's subordinate inquiries about Seok-ho's awareness of Jae-hyeon's final case. Director Kim offers a cryptic remark, hinting at Seok-ho's hidden agendas. Elsewhere, Seok-ho reflects on a poignant photograph depicting him, Jae-hyeon, and Director Kim, their past intertwined with secrets yet to be unraveled.
At his home, Hae-seong battles against his personal demons in solitude, yet he musters the strength to rejoin his team, vowing to press onward with their mission. He returns to the school halls, casually informing everyone that he had to attend to some personal affairs. However, Soo-a's demeanor towards him is even more irritable than usual.
After the final bell rings, Hae-seong makes his way to the grandfather clock housed in the student council room. With practiced ease, he opens the clock's casing and follows the cryptic instructions left by his father, which unravel the chilling tale encoded within the clock's mechanics. Guided by the tale of the upside-down corpse, Hae-seong adjusts the clock's hands to 6:30, the mirror image of midnight—the hour when it strikes. The clock responds by playing a haunting melody, which he promptly records on his phone.
Soo-a enters the room at this pivotal moment, engaging Hae-seong in conversation. She brings up the security guard, Kim Hyeon-ho, and accuses Hae-seong of concealing the truth about his encounters. A flashback transports them both back to the fateful night when Soo-a witnessed Hae-seong discovering Hyeon-ho's lifeless body and confronting a masked figure at gunpoint.
In the present, Soo-a confronts Hae-seong, revealing her awareness of his true identity—not as a mere student, but as an undercover NIS agent. Hae-seong comes clean, presenting her with an ultimatum: collaborate with the NIS or face imprisonment for betraying confidential information. He grants her a single day to make her decision.
Hae-seong and his team decipher the recorded melody, realizing it's the school's anthem, penned and composed by Seo Byeong-mun. This revelation leads Hae-seong to a piano, mentioned in the third instalment of the haunting tales, rumored to play by itself. When Hae-seong confides in his team about Soo-a, their anger simmers as they recognize the gravity of his threat to an innocent civilian, risking his own incarceration. Seok-ho sternly instructs Hae-seong to seek Soo-a's forgiveness without delay.
That evening, Hae-seong appears unexpectedly outside Soo-a's mother's cozy diner, his demeanor a mix of contrition and desperation. With heartfelt apologies, he pleads with Soo-a to guard their secret, weaving promises of reparation so enticing that Soo-a briefly visions herself receiving a presidential citation and a permanent position within the school's walls. Captivated by these visions, she impulsively agrees to aid Hae-seong in his enigmatic endeavor.
As they stroll down the moonlit street, Hae-seong confides in Soo-a, revealing that Yoo-jung is his cherished younger sister. He reaches out, clasping her hand warmly and vowing to shield her from any harm, a gesture that sends a wave of fluster through Soo-a.
Meanwhile, in a shadowy corner, Myeong-ju confronts a mysterious figure cloaked in darkness. This hidden advisor disapproves of her handling of Kim Hyeon-ho, a revelation punctuated by a flashback. In the flashback, Myeong-ju confronts Hyeon-ho with incriminating documents exposing his past as an NIS agent. Hyeon-ho, undeterred, announces his intention to entrust Hae-seong with his father's notebook. He further accuses Myeong-ju of being aligned with an NIS mole who seeks the elusive gold.
Returning to the present, Myeong-ju engages in a tense conversation with this very mole, who has instructed her to prevent any further bloodshed. She argues that Hae-seong's encounter with Hyeon-ho could have proved fatal for both of them.
Elsewhere, Hae-seong embarks on an investigation into the haunted piano that plays by its own accord. The narrative pivots around the president of the piano club and a young woman who once shared the ivories with him, until she discovered her pregnancy and was summarily rejected by him. Consumed by despair, she took her own life, and ever since, the piano has echoed with an otherworldly melody, a haunting testament to their tragic tale.
Currently, Hae-seong is taken aback by Soo-a's scream as she peruses the narrative over his shoulder. Her mind drifts back to the grand piano stationed in the chairwoman's office, accessible only during the exclusive event known as Night for Byeongmunites. Alas, Hae-seong's attendance is barred due to his accumulation of 20 demerit points stemming from his school absences. Frustrated, he confronts Soo-a about the situation, inadvertently hurting her feelings when she jestingly claims to be older than him.
That evening, Soo-a visits the NIS team's sandwich shop doubling as their headquarters, presenting a potential solution. She proposes that Hae-seong participate in a competition to become the next face of Byeongmun High, a feat that promises 25 merit points. They both venture to the contest, where Hae-seong initially struggles mightily with posing for photographs. With Soo-a's coaching, however, his performance transforms dramatically, earning him accolades.
Following Hae-seong's turn, Ye-na steps into the spotlight, throwing a tantrum when the photographer requests she roll up her sleeve. Yoo-jung intervenes, concocting an excuse to whisk Ye-na away. A flashback reveals Yoo-jung discovering scars adorning Ye-na's wrist. defense. Misinterpreting their interaction as romantic, Ye-na flees in distress, assuming they are a couple. Yoo-jung scolds Hae-seong for his interference.
Later, Soo-a counsels Hae-seong to allow Yoo-jung to seek him out when she desires assistance. Her words evoke memories of Hae-seong's childhood friend, Bong-ja. As they depart Soo-a's mother's diner, her mother calls her by the name Bong-ja, prompting Hae-seong to realize Soo-a's true identity.
In a closing scene, Hae-seong descends into the basement of the old school building, which is buried under piles of papers pertinent to the ongoing investigation. Guided by his father's notebook, he realizes this was his father's secret lair. He vows to locate his father soon.
