The curtain lifts on Episode 1 of "Undercover High School" with Jeong Hae-seong's poignant narration, unraveling the tale of how he found himself entangled in this intricate web. It all commenced with a cloak-and-dagger operation where a Korean smuggler exchanged a gleaming golden statue with a Chinese counterpart, with Hae-seong watching intently from his concealed vantage point. As soon as the statue came into view, he sprang into action, assisted by a colleague cleverly disguised as one of the smugglers' lackeys.
The Korean smuggler darted away with the precious artifact amidst the ensuing turmoil, and Hae-seong gave chase, first on foot, then aboard a bike. Despite receiving orders to await reinforcement, his determination propelled him onward, culminating in a daring leap onto the culprit's vehicle. Inside, a fierce battle ensued, culminating in the car flipping over and exploding in a fiery blaze. Miraculously, Hae-seong emerged unscathed, clutching both the seller and the statue.
Back at the National Intelligence Service headquarters, Hae-seong and his team eagerly unwrapped the statue, only to discover its arm was missing. Upon inspecting the debris, they realized that Hae-seong's bullet had ricocheted, shattering the statue. Team leader An Seok-ho broke the grim news to Director Kim of the police, who promptly demanded Agent Jeong's dismissal. Seok-ho, recognizing Hae-seong's invaluable skills, pleaded his case, offering to undertake any other task. Pleased with this compromise, Director Kim handed over a confidential file.
Later, the quartet drowned their sorrows in drinks, with Seok-ho revealing that Hae-seong could retain his position if he undertook the Kim assignment. The mission revolved around Byeongmun High School. During Japan's colonial rule, Seo Byeong-mun, the founder of the Byeongmun Foundation, had been bestowed a horde of gold bars by an emperor, intended for Korea's independence. However, Byeong-mun greedily kept it all for himself. As the nation won its freedom, he vanished into obscurity, with rumors abounding that the remainder of his gold lay hidden somewhere within the school's confines.Recently, the National Intelligence Service (NIS) received a covert tip-off suggesting that a clandestine transfer of gold bars was imminent, targeted from the confines of a school to an undisclosed location. Despite conducting a thorough search of the affiliated foundation, the NIS overlooked the school itself in their quest for the precious metal. The mission then fell to Hae-seong, tasked with infiltrating the high school undercover to uncover the gold. Though Hae-seong harbored reservations about posing as a student, there were no viable alternatives.
Elsewhere, the educators of Byeongmum High School gathered at a karaoke establishment to celebrate Vice Principal Baek's birthday. To ingratiate herself with him, Teacher Oh Su-a consumed an entire tumbler of soju, leading to a state of profound intoxication. In her intoxicated state, she inadvertently removed Baek's wig, causing a scene.
Back at the restaurant, Hae-seong came to the stark realization that the success of their entire operation hinged on his acceptance of this unusual assignment. After his colleagues departed, Oh Su-a stumbled in, revealing it to be her mother's diner. Suddenly, a group of rowdy men began to hassle the restaurant owner and later, Su-a herself when she stood up for her mother. Hae-seong intervened swiftly and effortlessly dispatched the troublemakers, earning the gratitude of both mother and daughter.
Returning home, Hae-seong pondered his predicament. A poignant flashback unveiled the bullying he had endured in high school, where he was mercilessly beaten and labeled an orphan by his tormentors. Compelled by these memories, he phoned Seok-ho and accepted the assignment. The team embarked on preparations, with Hae-seong immersing himself in the intricacies of the school's culture, including its rigid student hierarchy. His fake identity was meticulously crafted, complete with slang and jargon peculiar to the school's community.
Later, Hae-seong shared a meal with Seok-ho, his wife, and their daughter, during which it became apparent that Seok-ho and his wife had essentially raised Hae-seong after the loss of his father. Seok-ho revealed to his family that Hae-seong would be attending Byeongmun High, where his own daughter, Yoo-jung, was also enrolled. This twist added an unexpected personal layer to Hae-seong's mission.
The following morning dawned, and Hae-seong donned his school uniform, embarking on his journey to school with renewed determination. Su-a greeted him warmly, but her smile soon faded as she recognized him as the diner patron from the previous night. She reprimanded him sternly, yet Hae-seong expertly evaded any disciplinary consequences.
In the classroom, Ye-na eagerly expressed her desire for Yoo-jung to accompany her on the school's exchange trip to France. Just then, Su-a entered the room and instructed Hae-seong to introduce himself to the class. He took his seat beside a boy named Lee Dong-min, unaware of the drama that awaited him. Yoo-jung, taken aback, quickly sent him a text, summoning him to the school rooftop after classes. There, she pleaded with him to keep her academic struggles a secret from her parents.
As Hae-seong wandered the school grounds, his eyes caught sight of a gate adorned with a "Keep Out" sign. Curiosity piqued, he ventured inside only to find the entrance to a building firmly locked. Behind one of the walls, he discovered electricity meters, their readings raising further questions in his mind. Suddenly, the sound of bullying reached his ears – Dong-min was being harassed by a group of school bullies. Hae-seong was about to retreat when a noise drew their attention, compelling them to order him to fetch items for them. Just as he was about to confront them, a vision of Seok-ho flashed through his mind, urging restraint.
In the staff room, Su-a recounted Hae-seong's story to her friends, vowing to mete out justice if she caught him misbehaving again. As the school day drew to a close, Hae-seong inquisitively asked Dong-min about the forbidden building. Dong-min revealed that it was an old facility dating back to the Japanese colonial era, its history shrouded in mystery.
Elsewhere, Seo Myeong-ju, the chairwoman of the Byeongmun Foundation, attended the funeral of a man who had taken his own life. She solemnly promised the deceased's wife that the foundation would take care of their son. However, her gesture was tinged with controversy as an assemblyman later pointed out their long-standing rivalry.
At home, Hae-seong shared his suspicions about the old building with Seok-ho, speculating that the gold might be hidden there due to the unusually high electricity meter readings. Determined to uncover the truth, Hae-seong sneaked back onto campus under the cover of night, arming himself with a device to bypass the security cameras. With deft movements, he picked the lock to the old building and stepped inside. As Undercover High School Episode 1 drew to a cliffhanger, Hae-seong was in the midst of his search when a shadowy figure crept up on him, a gun pointed menacingly in his direction.
