The twelfth episode of "Undercover High School" kicks off with Hae-seong struggling to emerge from the wreckage of a car crash, only to hastily dial Soo-a's number from a grimy payphone. Through his perseverance, Soo-a locates him, swiftly escorting him to a hotel room where she tends to his wounds. As dawn breaks, Hae-seong confides in Soo-a, revealing that Seo Myeong-ju is responsible for his father's demise.
Back at the school's corridors, Myeong-ju receives shocking news: the school board intends to terminate her position. Meanwhile, in the hotel room, Hae-seong discovers a warrant for his arrest and promptly reaches out to his team for assistance. The team arrives, with Mi-jung revealing a crucial twist—she had indeed duplicated the recording of Myeong-ju's confession to Jae-hyeon's murder, but cleverly concealed it back at Hae-seong's residence.
Just as they devise a strategy to retrieve the incriminating evidence, the hotel manager's suspicion grows, leading to a call to the authorities. The police descend upon the hotel, forcing Hae-seong and Soo-a to make a daring escape through the window. In a moment of urgency, Hae-seong instructs Soo-a to shield the schoolchildren while he evades the pursuing officers.
Soo-a returns home, clutching a poster she crafted the previous night, a vivid protest against Myeong-ju's controversial reforms at the school. Simultaneously, Myeong-ju attends a crucial meeting at the Byeongmun Foundation, reminding the board that her dismissal requires the signatures of at least three-quarters of the school's students and staff. Elsewhere, Seok-ho, Mi-jung, and Young-hun find themselves incarcerated.
In a tense phone conversation, Hae-seong outlines his plan to infiltrate his home, but his weakened state from the crash catches him off guard, tripping over as he speaks. Despite this, he collaborates with Director Kim's junior agent, Gong Jin-sang, to secure the flash drive hidden in his house. A flashback interpolates the moment Hae-seong convinced Jin-sang of Myeong-ju's nefarious role, urging him to side with the truth.
Soo-a posts her poster with determination across the school walls. Myeong-ju discovers it and Soo-a confronts her head-on, vowing to go to any lengths to safeguard the students' well-being. Myeong-ju retaliates by offering to revoke all the controversial changes if Soo-a resigns from her position. Soo-a, without hesitation, accepts the deal. Meanwhile, Yoo-jung, who happens to eavesdrop on their conversation, catches every word. Unbeknownst to Soo-a, Myeong-ju harbors no intention of keeping her word and intends to push forward with her policies regardless.
Hae-seong, driven by a sense of justice, turns himself in to the police and presents the incriminating recording. This vital evidence not only confirms Jae-hyeon's murder but also unveils Principal Park's involvement in Director Kim's assassination, orchestrated by Myeong-ju. Hae-seong cautions Detective Cho, his handler, to mask any signs of collaboration with Myeong-ju.
To curry favor with the students, Myeong-ju convenes an assembly and falsely promises to abandon her contentious policies. Yoo-jung, unable to keep quiet, exposes Myeong-ju's manipulation that forced Soo-a's resignation. The revelation sparks a rallying cry among the students, who unanimously demand Myeong-ju's resignation. Soo-a, witnessing their unified support, is touched beyond measure.
The long-awaited arrest warrants for Myeong-ju and Principal Park finally arrive. Hae-seong and his team rush to the school, their resolve hardened. Principal Park, upon learning of the warrants, informs Myeong-ju, who instructs him to flee with Ye-na.
Myeong-ju dials Hae-seong's number, her voice dripping with ominous foreshadowing. As the sprinklers activate, they expel not water but gasoline, soaking the school in a hazardous bath. With a flick of her cigarette lighter, she holds the entire establishment hostage, threatening to incinerate it all.
Hae-seong arrives on the scene, his head pounding with renewed agony. He charges into the school, gun blazing, but Myeong-ju counters with a grim ultimatum, forcing him to surrender his weapon. Simultaneously, Yoo-jung sends an urgent message to Ye-na, who persuades Principal Park to turn the car around. Ye-na arrives, pleading with her mother to halt her crazy actions. Amidst her desperation, Myeong-ju considers taking her own life, but Soo-a intervenes,intervenes just in time. The lighter slips from Myeong-ju's grasp, but Hae-seong catches it at the eleventh hour, averting catastrophe.By that juncture, Seok-ho and his companions had arrived to apprehend both Myeong-ju and Principal Park. Just as the dust seemed to be settling, Hae-seong collapsed, only to awaken in a hospital bed later, diagnosed with a concussion. Days passed, and his teammates filled him in on the latest developments: Principal Park was shouldering the blame for everything, thereby clearing Myeong-ju of any wrongdoing.
Hae-seong sought out Park and unveiled a poignant revelation – Myeong-ju's family had taken in the orphaned Park as one of their own, forging a bond akin to family. He counseled Park to consider the welfare of both Myeong-ju and Ye-na in making his decisions.
During the subsequent trial, Park confessionally acknowledged all their collective misdeeds. Consequently, Myeong-ju was handed a life sentence. Time went by, and Ye-na visited her mother behind bars, disclosing her decision to quit school and embark on a fresh start, relying solely on her own talents this time around. She assured Myeong-ju that she needed some time to comprehend her and pledged to revisit.
Back at school, Hae-seong paid another visit to the students, only to receive a tepid reception. Undeterred, he shared his fondness for the time spent and his impending nostalgia. As he departed, a paper plane inscribed with "Thank you for being my friend" fluttered at his feet. Unexpectedly, a cascade of paper planes flew out of the windows, directed towards him, bearing the students' well-wishes.
Later, over a meal with Soo-a, she presented Hae-seong with the student council newsletter Jae-hyeon had compiled. The title, a synonym for "comet," struck her as a subtle message from Jae-hyeon to Hae-seong.
Hae-seong proceeds to tidy up the attic of his abode, uncovering a nostalgia-tinged memory. A flashback transpires, depicting Jae-hyeon thoughtfully arranging glow-in-the-dark stars in the configuration of the Big Dipper across the ceiling. Curious Si-hyun inquiries about the absence of the North Star, to which Jae-hyeon playfully replies that he's concealed it as a secret. Presently, Hae-seong stumbles upon the North Star sticker, lying forgotten on the cupboard floor. Beneath the floorboards, he discovers a cache of gold bars, presumably stashed by his father during his clandestine mission. With a sense of accomplishment, Hae-seong contacts Seok-ho, declaring his mission fulfilled.
The narrative leaps five years into the future. A youthful lass hops into Soo-a's car, where Soo-a is engrossed in a web novel titled "Undercover High School," penned by Lee Dong-min. This youngster is an old pupil from Soo-a's previous teaching stint, who had once extracted a promise from Soo-a to become a full-time educator. Now, Soo-a proudly displays her full-time teaching certificate to her former student, and they revel in this joyous milestone.
Elsewhere, Yoo-jung files an application to join the NIS, while Ye-na is immersed in her artistry, doing what fuels her passion. Seok-ho has seen a rise in his career. Young-hun and Mi-jung have finally consummated their love, and during a gathering at Soo-a's mother's diner, they reveal their joyous news of pregnancy to Hae-seong and the others. Hae-seong reflects deeply, realizing that the true treasure lies in these people and the cherished moments they share.
Later under the starlit sky, Hae-seong and Soo-a embark on a stargazing adventure. Hae-seong slips a ring onto Soo-a's finger, and their lips meet in a tender kiss. The closing scene of Undercover High School Episode 12 finds Hae-seong entering a classroom, announcing his new role as their class teacher. A shift in profession? A fresh covert assignment? The mystery remains!
