Episode 9 of "Undercover High School" kicks off with Soo-a stepping into Hae-seong's abode, only to stumble upon a masked intruder rummaging through his possessions, armed with a knife. In the sudden turmoil that ensues, the intruder strikes Soo-a before fleeing the scene. Hae-seong arrives shortly thereafter and rushes her to the hospital, where she undergoes treatment, though she remains unconscious.
Hae-seong's team converges at the hospital, yet his demeanor towards Seok-ho seems somewhat askance. He phones the school, feigning illness, prompting Ri-an to take charge of Soo-a's class, informing the students that Soo-a is entangled in personal affairs. However, one pupil, whose father runs the hospital, reveals to everyone that Hae-seong was the one who brought Soo-a there. Yoo-jung attempts to brush it off as mere coincidence, but later, as she walks alongside Ye-na, she mentions her rigorous studies for the impending math competition. This comment stirs an uneasy chord in Ye-na, who is now privy to the fact that the competition's results are tampered with.
At the hospital, Soo-a awakens, confiding in Hae-seong her suspicions about Lee Jun-ho. These suspicions are solidified when Young-hoon calls Hae-seong, informing him that the DNA sample found at his house matches Jun-ho's. Hae-seong vows to inform Seok-ho himself about this revelation.
Returning to his home, Hae-seong ponders over what Jun-ho could have been searching for and realizes that his father's notebook is missing, possibly hinting at a connection to Seo Myeong-ju. As fate would have it, Jun-ho is indeed seen sitting with Myeong-ju at school. He implores her to uphold her end of their agreement and shield him from both arrest and the cheating scandal that looms over his head.
As Myeong-ju flips through the notebook, she realizes with a start that the fourth ghost story remains a mystery, devoid of any details. Her mind drifts back to her conversation with Hae-seong in the cafeteria, sparking a hunch that he might hold the key to unraveling this elusive tale. Determined to uncover the truth without delaying Hae-seong's departure, she instructs Park Jae-moon to delve into the matter.
Ye-na enters the scene, her eyes pleading as she asks her mother, Myeong-ju, to ensure that the math competition this time will be a fair fight. After Ye-na's departure, Myeong-ju's resolve hardens; she makes it abundantly clear that she intends to uphold the competition's integrity in her own way.
Elsewhere, Young-hoon and Mi-jung witness an ominous scene: Principal Park escorting Lee Jun-ho to an abandoned car dealership. Without warning, Park disables Jun-ho with a swift blow and orders his men to place him in a car, their sinister plan to crush it and eliminate Jun-ho unfolding before their eyes. Intervening bravery sees Young-hoon and Mi-jung caught in the web, bound and trapped inside the vehicle themselves.
Just when all seems lost, Hae-seong arrives on the scene like a knight in shining armor, dispatching Principal Park and his henchmen with heroic prowess, securing the freedom of his friends. With Jun-ho in tow, Hae-seong brings him to Soo-a, who demands an explanation for his involvement in the cheating scandal. Jun-ho claims it was Myeong-ju's doing, prompting Soo-a to insist on rectification. Hae-seong then whisks Jun-ho away to a hidden safe house, instructing him to remain indoors until further notice, taking possession of his phone as a precaution.
Hae-seong contacts Seok-ho, outlining his strategy to keep Jun-ho confined in the safe house until 7 AM the following day, at which point Jun-ho will be escorted to the police, thereby bringing Myeong-ju's scheme to its knees. While telling Seok-ho he's heading home, Hae-seong secretly remains vigilant outside the safe house, his actions appearing as a subtle test of Seok-ho's loyalty and trustworthiness.
A poignant flashback unfolds, depicting Seok-ho discovering the tiny Si-hyun wandering alone on the street and sternly scolding him for his incessant propensity to flee. Si-hyun, with tears in his eyes, confesses his orphan status, prompting Seok-ho to soften and offer him a comforting embrace, vowing to be his fatherly figure until his real dad returns.
In the present day, Hae-seong breathes a sigh of relief as the night passes without anyone claiming Jun-ho. The next morning, he phones Seok-ho, his voice laden with apology. Seok-ho, however, reveals his omniscience and playfully suggests a meal as compensation.
Myeong-ju steps into her school office, startled to find Hae-seong awaiting her. His piercing gaze demands an explanation for his selection in the operation, but she remains tight-lipped. Hae-seong then produces a newspaper clipping, showing Jun-ho being apprehended by the police. From his pocket, he pulls out a flash drive, Jun-ho's gift, containing incriminating evidence that Myeong-ju orchestrated the cheating scandal. Soon, the police arrive, and the scandal is splashed across the news, leading to Park Jae-moon's arrest as well.
On the school rooftop, Hae-seong confronts Soo-a, his tone filled with gratitude. He tells her that, thanks to her courage, the school's atmosphere has undergone a remarkable transformation. Meanwhile, at the police station, Director Kim visits Myeong-ju, his words like a dagger as he highlights the impending collapse of her Edu-city project. He makes it clear that he has no obligation to assist her now. In response, Myeong-ju laughs maniacally, challenging Kim with a question that echoes her despair – what leverage does a desperate woman possess when she has lost everything?
Later, Hae-seong and Seok-ho share a meal, the ambiance lightened by the revelation that Seok-ho was unaware of Hae-seong's suspicions towards him. Hae-seong opens up, confessing his initial suspicion that Seok-ho was in cahoots with Myeong-ju. Yet, he now harbors doubts about Director Kim's innocence.
Seok-ho confesses that on the fateful day Hae-seong's father vanished, he received a call from him, hinting at some revelations about Director Kim. Yet, hesitant and running late, Seok-ho arrived at the designated meeting spot to find it empty, Hae-seong's father never to be seen again. Haunted by the fear of Hae-seong's resentment, Seok-ho kept this secret buried. Hae-seong, however, reassures him, stating that their bond is unbreakable, too strong for such trivial misunderstandings.
Returning to the school halls, Hae-seong seeks Dong-min's insight into the enigmatic final tale, only to discover that Dong-min knows little beyond the fact that those who uncover it seem to vanish into thin air. Meanwhile, in the staff room, Vice Principal Baek announces the impending visit of a vice-commissioner from the Department of Education, appointing Soo-a as his guide.
Elsewhere, whispers spread about Ye-na's unforeseen advantage from the cheating scandal, leaving her anxious and unwittingly scratching at the scars adorning her arms. Noticing her distress, Soo-a offers comforting words, reminding Ye-na that she bears no culpability for the events that transpired.
Yoo-jung approaches Hae-seong, expressing gratitude for his pivotal role in transforming the school's atmosphere. As they stroll, Hae-seong's eyes catch a glimpse of a note on the school bulletin board, suddenly realizing that the "bead" clue unearthed by Soo-a could indeed be a reference to Seo Byeong-mun himself.
In the classroom, Soo-a vows to safeguard her students from any recurrence of the cheating scandal, vowing to go above and beyond. Learning of her dual responsibility of entertaining the vice-commissioner and overseeing the school's cleanliness, her students are moved to lend a helping hand, collectively transforming the school's ambiance through their collective efforts.
The subsequent day dawns with the arrival of several officials, among them, to everyone's astonishment, Seo Myeong-ju. Hae-seong dashes to the lobby upon hearing the news, only to be confronted by Myeong-ju, who tears off his uniform badge and urges him to cease his deceptive charades.
The final scene of Undercover High School Episode 9 unfolds with Soo-a awakening in a hospital bed, her gaze falling upon Hae-seong, peacefully slumbering beside her. A tender smile graces her lips as she gently brushes her fingers through his hair, quickly feigning sleep as his eyes flutter open.
