The second episode of "Undercover High School" kicks off with an exhilarating scene where Hae-seong, caught in the act of exploring the enigmatic old building on campus, finds himself confronted by Su-a, who dramatically points a gun at him. To his relief, it's merely a tear gas gun, which she wields as part of her patrol duties. With quick reflexes, Hae-seong manages to deflect her aim and bolts away, leaving her behind in astonishment.
Later, Hae-seong recounts the thrilling encounter to Seok-ho, while Su-a narrates her version to her mother. As their paths converge on the way home, a curious coincidence reveals that their houses are merely a stone's throw apart.
Back at school, Hae-seong is handed a video game on a mobile phone by a trio of bullies, including a towering figure named Park Tae-soo, who issues a dare: reach the final level or face consequences. Meanwhile, on the rooftop, Yoo-jung intercepts Hae-seong with a stern warning to be vigilant of Tae-soo's antics.
Inside the staff room, Vice Principal Baek metes out punishment to Su-a with mundane chores for her failure to apprehend the intruder. Elsewhere, Seo Myeong-ju, a figure of formidable authority, gets wind of the intruder's escapade. Her trusted lieutenant, Principal Park Jae-moon, informs her that a video jammer disrupted the CCTV footage, shrouding the incident in mystery. She instructs him to lure the intruder back to observe his next moves.
The whispers of the old building's past echo through the corridors, even among the teachers. Min-ji, Su-a's colleague, shares eerie tales of a forgotten ballet studio lurking in the basement's depths. Just as the tales peak, Vice Principal Baek strides in, catching Su-a off guard as she accidentally spills her drink over him, incensing him further and adding another layer of tension to the already charged atmosphere.As class resumed, Su-a observed Dong-min's absence and inquired about his whereabouts. Tae-soo casually mentioned illness, but Hae-seong, ever perceptive, sensed something amiss. Later, he witnessed Jae-moon sneaking into the decrepit old building and, driven by curiosity, decided to follow. However, his efforts were thwarted by a vigilant security guard, who grumbled about the owners' overly protective stance towards the decrepit structure.
Hae-seong swiftly informed his team of his findings. Just as he was about to hang up, a rustling noise emanated from a nearby dustbin. To his surprise, he discovered Dong-min bound and concealed within. After liberating Dong-min, the latter pleaded with Hae-seong to keep his plight from the teachers, fearing it would exacerbate his situation. Their conversation was interrupted by Su-a's arrival, who promptly summoned Hae-seong to her office.
In her office, Su-a interrogated Hae-seong about whether he was bullying Dong-min. A sudden realization struck her—Hae-seong's eyes bore a striking resemblance to those of the intruder she had encountered earlier. She attempted to scrutinize him further, but Hae-seong evaded her gaze, causing her to stumble.
Back at the NIS, the team was captivated by a news report detailing Seo Myung-ju's ambitious Edu-City Project, a colossal undertaking valued at 800 million—a figure that conveniently coincided with the worth of the hidden gold bars.
The student council meeting was tense as Yoo-jung confronted Tae-soo over his boastful admission of tying up Dong-min. Tae-soo reminded Yoo-jung of her former insignificance until Ye-na, Myeong-ju's daughter, befriended her. Unexpectedly, Ye-na sided with Yoo-jung and leveraged her father's influence, threatening Tae-soo's father's position as an assemblyman unless he apologized to Yoo-jung. However, Ye-na later cautioned Yoo-jung against such outbursts, asserting that she would handle such matters herself.
Elsewhere, Seo Myung-ju instructed Principal Park Jae-moon to introduce a special class, offering unparalleled privileges to its members. When Jae-moon expressed concerns about the growing discontent regarding the preferential treatment accorded to the top 10% of students, Myung-ju instructed him to create further divisions among the student body, fostering competition and rivalry.During their lunch break, Hae-seong finds himself seated alongside Dong-min, who captivates him with tales of the eerie happenings within the school's ancient structure. Dong-min relates a chilling story of a ballet student, relentlessly bullied by her peers. In a desperate bid to escape their torment one day, she stumbled into a fateful accident that robbed her of her legs, her passion for dance extinguished. Unable to cope with her loss, she took her own life, hanging herself in despair. Her bullies, driven by guilt, concealed her body, and ever since, her spirit allegedly haunts the basement, seeking justice.
Dong-min, claiming to have witnessed the spectral apparition, pulls out a video from his device, revealing a shadowy figure gliding through the ballet studio. Hae-seong's eyes are drawn to a digital lock adorning one of the doors in the footage, just as Dong-min prepares to unveil another haunting tale. However, their moment is abruptly shattered by the arrival of Tae-soo and his cronies, who promptly resume their bullying of Dong-min.
Unable to stand by any longer, Hae-seong rises to Tae-soo's challenge, engaging in a fierce confrontation. Though Tae-soo provokes a fight, Hae-seong emerges victorious. The commotion attracts Su-a, who summons both to the counseling office. The vice principal escorts Tae-soo away, while Su-a delves into questioning Hae-seong. Hae-seong wonders aloud if Su-a turns a blind eye to Tae-soo's antics out of ignorance or complicity, querying her genuine concern for her students.
Su-a is deeply affected by Hae-seong's words, leading her to seek solace in alcohol at her mother's diner once more. Meanwhile, at the NIS office, Seok-ho expresses his confidence in Hae-seong's capability to execute their mission to his colleagues when Director Kim bursts in with a video depicting Hae-seong striking Tae-soo.
Elsewhere, Tae-soo finds himself in the company of an elderly man, who informs him of a debt that needs settling. In lieu of cash, the man suggests Tae-soo procure a few ID cards as repayment. In a different part of town, Hae-seong and Seok-ho confront Director Kim, who rebukes them both sternly. The director also makes a pointed remark about Hae-seong following in his father's footsteps at the NIS, cautioning him not to dishonor his legacy. A nostalgic flashback transpires, depicting young Hae-seong bidding farewell to his father on the eve of his birthday as the latter embarked on a mission.
Elsewhere still, Tae-soo arrives at Dong-min's residence, weaving a tale to deceive Dong-min's grandmother into handing over her ID. Meanwhile, Dong-min, occupied with his part-time job delivering food, catches a glimpse of Hae-seong sneaking back into the school, adding another layer of intrigue to this already tangled narrative.
He manages to catch up with Hae-seong, convinced that his friend is merely testing his resolve with this ghostly tale. Ignoring Hae-seong's vehement protests, he pushes ahead and enters the eerie building. The sound of peculiar noises echoes from the ballet studio, drawing their attention. Peering inside, they witness a mysterious man clad in a hat, frantically driving nails into one of the floorboards. Catching sight of their intrigued gazes, the man abruptly flees the scene.
Determined to uncover the truth, Hae-seong advances to the locked door and employs the clever device bestowed upon him by Seok-ho to gain entry. Upon stepping inside, he is greeted by a room filled to the brim with books and papers. A meticulously detailed investigation covers the wall, revealing the intricate layers of a hidden mystery. With a sense of urgency, Hae-seong dials Seok-ho and informs him that it appears Myeong-ju is also hot on the trail of the elusive gold bars.
Elsewhere, Dong-min's curiosity leads him to lift the very floorboard the shadowy figure had been working on. To his astonishment, a lifeless body lies concealed beneath. Meanwhile, in a clandestine corner, Myeong-ju's eyes peer through a hidden camera, her gaze fixed intently on Hae-seong as he stands in her room, unaware of her silent scrutiny.
