Episode 5 of "Undercover High School" kicks off with an intense scene where Seok-ho, Mi-jung, and Young-hoon engage in a heartfelt prayer session, their thoughts collectively focused on Hae-seong's upcoming midterm exam success. A sudden flashback transports us to a moment where Soo-a is left utterly astonishment as Hae-seong's team tumbles out of his cupboard, clad in an elaborate ruse as his supposed part-time job colleagues aiming to orchestrate a grand surprise for him. However, Soo-a's skepticism remains steadfast. In a dramatic twist, Hae-seong pretends to collapse, prompting his comrades to hastily "rush" him to the hospital.
As the present unfolds, Hae-seong successfully concludes his exam, while Young-hoon remains deeply engrossed in the afterglow of his unreciprocated kiss with Mi-jung. He confessionally reveals his feelings to her, but her response is anything but favorable. Their subsequent encounter with Seok-ho and Hae-seong over dinner only adds to the complexity of their emotions. Seok-ho's mind is still cluttered with fears of Soo-a's lingering suspicions, while Hae-seong finds himself constantly reminiscing about his unplanned kiss with Soo-a.
Meanwhile, Soo-a's suspicions towards Hae-seong's true identity as an adult continue to linger. As she exits his building, her eyes catch a glimpse of his phone number on his car, sparking her curiosity. Over a few drinks with her colleague Ri-an, she shares her findings, unaware that her mother is eavesdropping on their conversation. That fateful night, a tipsy Soo-a ends up at Hae-seong's doorstep instead of her own. In a desperate attempt to divert her, Hae-seong tries to disguise his voice as a woman over the intercom, but fortunately, Soo-a's mother intervenes, escorting her back to their house. To Hae-seong's astonishment, he realizes they are next-door neighbors.
The following day, the midterm results are announced, revealing Hae-seong's impressive tenth-place ranking, a triumph in achieving his personal goal. To everyone's surprise, Yoo-jung secures the top spot, unseating Ye-na for the first time. Despite Ye-na's apparent nonchalance, her inner jealousy is palpable. Elsewhere, Chairwoman Myeong-ju meets with Chief Editor Yoon of the Seojin Daily, proposing a mutually beneficial arrangement where he pens an article about her vision for a global privatized school. However, Yoon, unyielding to bribery, declines her offer and exits abruptly. On her return, Principal Park informs Myeong-ju of Ye-na's ranking change, leading to a tense scene where Ye-na receives a summon from Myeong-ju, her anxiety evident as she steps into the office.
Hae-seong announces to his team that he has submitted his application for the student council, albeit with two other competitors vying for the spot. In an attempt to curry favor with Ye-na, he employs clumsy, archaic gestures, much to everyone's amusement, yet somehow still manages to secure a place on the council.
Meanwhile, Soo-a endeavors to reach Hae-seong's former school using his official records. The phone number, however, redirects to Hae-seong's team, who swiftly come to his aid by deflecting any inquiries. As Hae-seong enters the staff room to inquire about the key to the student council room, he catches a glimpse of Soo-a scrutinizing his records. In a bid to smooth things over, he then attempts to appease Yoo-jung, who steadfastly refuses to engage with him. Unfortunately, this exchange is witnessed by Ye-na.
Later, Seok-ho cautions Hae-seong to steer clear of Soo-a as much as possible.
The following day, Hae-seong attends his inaugural student council meeting. After the session concludes, he lingers around to examine the grandfather clock, only to be interrupted by Ye-na and Yoo-jung. Ye-na invites both of them to join her for dinner to discuss a matter of importance.
Elsewhere, Ri-an visits the sandwich shop where Hae-seong's team is ostensibly "working." She employs her camera to relay a live feed to Soo-a, who remains skeptical of Hae-seong. Naturally, their surveillance uncovers nothing unusual. Soo-a is subsequently burdened with additional tasks by Vice Principal Baek.
Yoo-jung meets Ye-na at an upscale restaurant, where they are shortly joined by Hae-seong. As their conversation unfolds, Hae-seong observes how Ye-na treats Yoo-jung like a mere servant, bossing her around with impunity. He calls Ye-na out on her behavior and takes the liberty of footing the bill, abruptly terminating their dining engagement. After Ye-na's departure, Hae-seong realizes this is the reason Yoo-jung was hesitant about his council involvement. Yet, she insists that she prefers the status quo and, tears welling up, begs Hae-seong not to interfere.
Ye-na returns to her abode, where she uncovers a revelation: it was her mother who instigated the inclusion of Hae-seong in the council. Meanwhile, under the shroud of night, Hae-seong ventures back to the school, driven by a desire to scrutinize the enigmatic grandfather clock. With his key, he unlocks its secrets, only to find the interior seemingly barren. Just as he ponders this discovery, a slip of paper slides beneath the door, bearing the chilling greeting, "Hello, Agent Jeong Hae-seong."
Hae-seong and Seok-ho present this cryptic message to Director Kim, who promptly instructs them to trace its origin. He subsequently tasks his secretary with investigating the school's staff. Seok-ho, concerned for his companion, exhorts Hae-seong to exercise caution throughout this perilous mission.
The following day, Director Kim delves into the records of the school's personnel, uncovering a startling revelation that leaves him breathless. On another front, Hae-seong learns that the CCTV footage around the council room has been erased during a server reboot, an act orchestrated by Principal Park on Myeong-ju's behalf. Yet, the identity of the individual who slipped Hae-seong the note and their motive remain shrouded in mystery.
Elsewhere, Soo-a finds solace in her colleague Joon-ho's assistance with the mountains of reports imposed by Vice Principal Baek. During this collaborative endeavor, her eyes chance upon a book documenting the 2003 student council members, sparking an unexpected revelation.
At his residence, Hae-seong receives a call from an unknown number, which transpires to be the school's security guard—the same one Hae-seong had caught encroaching upon the basement in earlier episodes. The guard, who appears to be the note's author, urges Hae-seong to rendezvous with him atop the school's roof.
Obeying the summons, Hae-seong makes his way to the school, only to be startled by the grandfather clock's sudden chiming. Dong-min's haunting tale and the ominous phrase "crimson eyes will come for you" echo in his mind. Before he can step inside, a figure plummets from the roof—the security guard, his eyes glowing an unsettling red. Clutched in his hand is a tiny black notebook adorned with stars, a flashback confirming it as the same one Hae-seong had bestowed upon his father years ago.
The episode concludes with another reminiscence, depicting a heartwarming meal shared by Si-hyun (Hae-seong's past identity), his father, Bong-ja, and others. Moreover, we witness Soo-a's astonishment upon discovering a photograph in the student council book—one featuring Si-hyun's father amidst the council members, adding another layer of intrigue to the unfolding narrative.
