Episode 4 of "Undercover High School" kicks off with Hae-seong on a mission to track down Gu Sang-tae, the notorious kingpin behind the clandestine gambling operation. With deft precision, he neutralizes the cronies and renders Sang-tae unconscious. Just then, Young-hoon, Seok-ho, and Mi-jung arrive to lend a hand, with Seok-ho reprimanding Hae-seong for keeping the bullying ordeal under wraps. He firmly asserts that they are a united front and Hae-seong should rely on them.
Hae-seong escorts Sang-tae to the school's anti-bullying committee, where the latter spills the beans about the entire racket, revealing that Tae-soo is a VIP member owing him a debt, payable in ID cards rather than cash. Hae-seong meticulously enumerates Tae-soo's criminal escapades, prompting Dong-min to finally voice his concerns. Hae-seong then produces a flash drive containing CCTV footage that unmistakably ties Tae-soo to Sang-tae.
In a subsequent scene, Myeong-ju informs Tae-soo's father of the inevitable expulsion due to his son's entanglement in a criminal saga. Despite the father's attempt to coerce her, it falls on deaf ears. Principal Park, acting on Myeong-ju's behalf, initiates a physical confrontation with the father. Later, Myeong-ju urges Park to expedite their search for the missing gold bars.
Soo-a later conveys the news of Tae-soo's expulsion to Hae-seong, expressing gratitude for his online post that exposed the truth. As Hae-seong gently removes a petal from her hair, Soo-a's heart skips a beat, fluttering with unspoken emotions.
The narrative transitions into the second chilling ghost tale. It revolves around a young student who perpetually ranked second in her class, overshadowed by her best friend who always secured the top spot. Driven by jealousy, she commits a heinous act, pushing her friend off the rooftop to a fatal demise. Later, while studying alone in the school, the clock strikes, and she is haunted by a specter with piercing red eyes, forever consumed by her dark deed.
Dong-min recounts the tale to Hae-seong, painting a picture of four enigmatic stories, the fourth remaining shrouded in mystery, unknown to all. They find themselves indulging in a meal at Hae-seong's team's quaint sandwich shop, where Hae-seong plays the role of a part-time worker with convincing charm.
Elsewhere, Ye-na and Yoo-jung embark on a shopping spree, with Yoo-jung bearing the brunt of Ye-na's retail therapists dreams, toting an array of shopping bags. As Yoo-jung notices Ye-na's growing fascination with Hae-seong, her own observations take shape.
Meanwhile, Oh Soo-a finds herself adrift in a sea of fluttering emotions, her mind replaying snippets of her childhood sweetheart. Caught in a compromising position—snoozing at her desk—by the vice principal, Soo-a's fate takes a twist as she's appointed the advisory teacher for the student council.
Hae-seong attempts to unlock the secrets of the grandfather clock once more, but to no avail. Enter Yoo-jung, pasting a vibrant poster for student council recruitment onto the noticeboard. While lending a hand with hanging photographs, Hae-seong stumbles upon a revelation: a mirror image of the grandfather clock, residing in the very chamber where the student council convenes. Ye-na, chance witness to Yoo-jung and Hae-seong's artistic endeavors, casts a wary glance, sensing a deeper intrigue.
When Hae-seong ventures to enter the council room, Ye-na suggests he join their ranks, offering him an extravagant gift she had splurged on. Hae-seong, conscious of her generosity misspent, gently scolds her for her lavish expenditure and urges her to invest in textbooks instead, steering her towards practicality.
Hae-seong approaches Soo-a with curiosity, inquiring about the prerequisites for becoming a student council member. She responds, outlining the stringent criteria: securing a ranking of 10th or better in his final examination, collecting 100 endorsement signatures, and obtaining the blessing of the student council president, Ye-na.
Upon further investigation, Hae-seong discovers that Dong-min has forged a bond with Tae-soo's two henchmen, who, having apologized to him, now grant him the liberty to strike them with a bat. Attempting to leverage this newfound connection, they endeavor to aid Hae-seong in amassing the required signatures, yet their efforts fall flat. Hae-seong faces a daunting challenge, his academic performance being lackluster and the midterms looming just a week away.
One bright morning, Hae-seong arrives at school to a delightful surprise – his team members have organized a free food stall and a lively dance performance, all in a bid to secure him the elusive 100 signatures. Inside the student council chambers, Ye-na expresses enthusiasm for Hae-seong's potential inclusion, while a fellow council member casts aspersions on him, accusing him of betrayal for his role in Tae-soo's expulsion. Ye-na is taken aback to learn that Yoo-jung opposes his entry with fervent conviction.
Yoo-jung confronts Hae-seong atop the school roof, sternly warning him to steer clear of the student council. With a hint of desperation, she threatens to expose his true identity if he persists. Hae-seong attempts to engage her in further conversation, but she remains stone-faced and unyielding.
Elsewhere, Myeong-ju instructs Principal Park to keep a close watch on Hae-seong and the clocks, a task she undertakes with utmost vigilance. Ye-na greets them both as she enters, and after Park's departure, Myeong-ju offers Ye-na words of encouragement, urging her to study diligently. She hands her ADHD medication, emphasizing the importance of ranking first to carve a lasting impression.
The following dawn, Hae-seong receives an unexpected call summoning him for a one-day military training session, oblivious to his covert mission. He contacts Soo-a, feigning illness and requesting a day of rest. That very night, Soo-a phones him back, insisting on bringing medicine to his doorstep.
However, Seok-ho and his team are yet to establish Hae-seong's house at the fictitious address. In a mad dash, they scurry around, ultimately scavenging objects and essentials from adjacent dwellings to transform the barren apartment into a semblance of a home. Soo-a arrives unexpectedly, prompting the trio to scurry into the closet for concealment. She hands Hae-seong a bowl of porridge, but he clumsily spills it on her, compelling her to seek refuge in the bathroom. Just as she prepares to depart, she stumbles and crashes to the ground, pulling Hae-seong down with her in an unanticipated lip-lock.
Concurrently, Young-hoon is struggling to suppress a sneeze within the confines of the closet. Mi-jung, in an attempt to silence him, plants a kiss on his lips, but this gesture only serves to make him faint and topple out of the closet, thereby exposing the entire team to Soo-a's astonished gaze.
The climactic scene of Undercover High School Episode 4 depicts Soo-a flipping through a photo album. Her eyes light upon a snapshot featuring her and another schoolmate, a boy who turns out to be none other than Si-hyun, Hae-seong in his youthful days. Flashbacks reveal that she was once known as Bong-ja, and the memory resurfaces with her coaxing Si-hyun to eat his peas, painting a vivid picture of their childhood bond.
