The seventh episode of "Undercover High School" kicks off with Hae-seong and Soo-a exiting her mother's diner, only to have her mother affectionately call her by her former moniker, Bong-ja. This revelation strikes Hae-seong like lightning; he recognizes her instantly from their shared childhood. Misinterpreting his amused smile as derision aimed at her old name, Soo-a storms away in a huff.
Later, at his abode, Hae-seong stumbles upon an aged photograph, capturing a youthful moment between him and Soo-a. A tender flashback ensues, showing a little Soo-a innocently proposing marriage to him.
Elsewhere, Ye-na finds herself in another scrape with her mother, who discovers Ye-na's self-harm scars. Meanwhile, Myeong-ju deliberately cuts her own arm, using it as a poignant reminder to Ye-na that their physical suffering is inconsequential in their quest to embody the school's image.
The next morning, Hae-seong catches Soo-a dancing enthusiastically to a tune in the staff room, her efforts to accumulate more points for her evaluation as a full-time teacher evident in her awkward yet determined moves. Embarrassed yet appreciative of her hustle, Hae-seong offers her commendation. This gesture prompts Soo-a to inquire if Ri-an thinks Hae-seong's kindness might signify romantic interest. Ri-an dismisses it as mere friendliness.
The teachers are then assigned the responsibility of selecting the school's new face. When the vice principal shows inclination towards another male student, Soo-a passionately advocates for the teachers' vote, inadvertently giving Hae-seong a fighting chance. Amid the confusion, however, Soo-a mistakenly marks the other student's name instead of Hae-seong's. To everyone's surprise, including Soo-a's, Hae-seong emerges victorious.
Amid his celebration, Hae-seong encounters Yoo-jung and attempts to engage her in conversation. He offers a heartfelt apology for meddling in her life. Yoo-jung, in turn, asserts that Ye-na is not the malicious girl she appears to be; rather, she is grappling with a challenging life circumstances.
Hae-seong is abruptly summoned away by his team, hastily sending Soo-a a brief message conveying his unexpected early departure. His team then drops a bombshell: the detective assigned to investigate Kim Hyeon-ho's demise must have orchestrated a cover-up, depicting it as a suicide. Their evidence hinges on the fact that the deceased's ailing daughter was transferred to a posh hospital under the Byeongmun Foundation's umbrella.
Switching gears to the third ghostly tale, Mi-jung reveals that the subsequent clue necessitates two individuals to play the piano in Myeong-ju’s office. Soo-a, entering the café with a confident stride, assures them that mastering the piece is no herculean task; she volunteers to tutor Hae-seong. Though Hae-seong is less than thrilled, Seok-ho mischievously dubs Soo-a a covert field agent, to which she responds by pleading for his assistance in securing a permanent teaching post.
Hae-seong drags Soo-a aside, sternly warning her against such impromptu actions. Curiosity piqued, he inquires about her relentless pursuit of a full-time teaching gig. Her answer unfolds with heartfelt sincerity, revealing it's for the sake of one of her former students. Reluctantly, Hae-seong caves in.
Later, at school, a clandestine operation unfolds as Soo-a and Hae-seong devise a plan to procure Principal Park’s fingerprint—the key to unlocking Myeong-ju’s office. Soo-a deftly procures the print by intercepting a book Park intended to check out from the library. Hae-seong expertly lifts the fingerprint, after which Soo-a initiates their piano lessons. However, Hae-seong finds himself continually distracted by her presence. In an attempt to regain focus, he demonstrates a few self-defense maneuvers, bringing them unexpectedly close.
Seok-ho proceeds to escort Soo-a to the NIS office, honoring her with the status of an agent, while concurrently informing Hae-seong of the temporary closure of their sandwich shop. The group then delves into strategic discussions about infiltrating Myeong-ju's office during the Night for Byeongmunites, settling on a plan to strike while she indulges in tea with the students' parents.
The much-anticipated evening arrives. Young-hoon assumes the guise of a waiter, blending seamlessly into the festive atmosphere. Hae-seong and Soo-a make their entrance, with Hae-seong unable to tear his gaze away from Soo-a's enchanting presence. A nostalgic flashback takes us to their childhood, where a youthful Hae-seong, innocent and earnest, vows to propose to Soo-a one day when she dons a stunning gown, to which she playfully agrees, setting the stage for future endeavors.
In the present, Hae-seong discreetly administers to Soo-a an injection capable of inducing temporary unconsciousness, a precautionary measure for unforeseen circumstances. He also bestows upon her a necklace embedded with a camera, reassuring her of his unwavering commitment to her safety.
Meanwhile, Myeong-ju gracefully introduces a liquor, a personal concoction of Chairman Seo Byeong-mun, to her guests during the tea session, intending to unveil it at the grand opening of Edu-city. She further surprises them by presenting each with an exclusive black card.
Elsewhere, Yoo-jung notices Ye-na's self-inflicted injury and attempts to lend a helping hand, only to be met with insulting epithets and slaps. This time, however, Yoo-jung refuses to cower, striking back with a resolute slap of her own.
During the tea ceremony, just as Myeong-ju prepares to share the Chairman's liquor, Young-hun executes his plan, plunging the area into darkness. Seizing the opportunity, Hae-seong and Soo-a make a dash for the stairs, only to be momentarily thwarted when Principal Park, sensing something amiss, orders security to the office. However, their progress is halted as the principal restores power almost instantly.
Young-hun skillfully assumes the role of a parent, orchestrating a commotion within the security room, cleverly obscuring Hae-seong and Soo-a from the prying eyes of the surveillance cameras. However, just as they make their way to the office, the security guards arrive on the scene. Undeterred, Hae-seong and Soo-a deploy their expertise to expertly neutralize the guards. Meanwhile, Mi-jung strategically heads to the security office, planting a kiss on Young-hun to divert the guards' attention away from the monitors.
Inside the office, Hae-seong and Soo-a sit down at the piano, their fingers dancing across the keys. Hae-seong notices with concern that one of the notes fails to resonate. As their musical piece concludes, there's an anticlimactic silence, prompting Seok-ho to command their departure. While erasing their presence from the room's records, Hae-seong stumbles upon a file labeled "undercover high school," revealing the intimate details of their operation, along with his personal profile.
A memory from Hae-seong's initial day at the NIS unfolds, where Seok-ho inquiries about his preferred name. A heartwarming flashback depicts a young Si-hyun sharing with his father his desire to be called Hae-seong—a name symbolizing comets, a testament to his love for the stars.
The final scene of Undercover High School Episode 7 cuts to Hae-seong's piano lesson, where Soo-a casts a tender glance his way, her lips curling into a knowing smile, encapsulating the intricate dance of secrets and passions unfolding amidst their clandestine mission.
