In Episode 5 of Gangnam B-Side, Commissioner Moon expresses his desire for Major Crimes to take charge of Joon-seo's case. Dong-woo, amidst his desperate search for Ye-seo, instructs Ji-su to manage the situation. His only clear directive is the urgent need to secure Jae-hee's video.
Meanwhile, Ye-seo's captor, Choi Su-in, reveals a familiarity with Jae-hee. Oblivious to Jae-hee's Jenny alter ego, Ye-seo pleads innocence, asserting her intentions to merely assist. Su-in remains unfazed, consigning Ye-seo to a room shared by other escorts.
A nostalgic flashback transports us to Ye-seo's high school days, where she was a target of relentless bullying. Jae-hee intervenes and saves Ye-seo in trouble, although somewhat annoyed by Ye-seo's inability to confront her tormentors. Empowered by Jae-hee's bravery, Ye-seo seeks her mentorship in becoming stronger. The scene shifts abruptly to a rooftop confrontation, where Ye-seo, despite her efforts, falls short in defending herself. Jae-hee's timely arrival scatters the bullies, but tragedy strikes as the ringleader lunges at Ye-seo, losing his footing and plummeting to his death. Plagued by guilt, Ye-seo finds solace in Jae-hee's reassurance that the bully's aggression was his undoing.
In the present, Dong-woo's relentless pursuit leads him to trace Ye-seo's phone signal to a building housing several vans belonging to Hangil Cleaning Service, contracted by Gangnam Trading Company for VIP event clean-ups.
Elsewhere, Gil-ho awakens, still bearing the brunt of his recent injuries. His fragile state is interrupted by回忆s of Dong-woo's urgent command to flee, promising to safeguard Jae-hee. The narrative cuts to Dong-woo visiting Jae-hee, who has regained consciousness. He updates her on Jin-young's rehabilitation and Gil-ho's disappearance, but Jae-hee insists he keeps the Colosseum a secret from Ye-seo to spare her worry. Instead, Dong-woo requests the video to "protect" her, to which Jae-hee subtly deflects, asking for some sustenance instead.
At the bustling store, he finds her charming as she whimsically selects an assortment of snacks and instant ramen. Ji-su's call interrupts the serene moment, her voice laced with worry for Ye-seo, while Jae-hee inadvertently eavesdrops. She hastily instructs Dong-woo to keep a close watch on Ye-seo before abruptly vanishing. In his diligent search for her, Ye-seo surrenders herself to Su-in, who promptly liberates Ye-seo from her confines. Meanwhile, Su-in confronts Jae-hee with a fierce scuffle, while K, the sleazy pimp who had summoned Dong-woo, scrutinizes the enigmatic video. He issues a stern warning, cautioning her never to defy Mr. Choi and immediately assigns her to laborious tasks.
As Su-in meticulously dresses Jae-hee, it becomes evident that they share a bond of friendship, and her actions were devoid of malice. Su-in, distressed by Jae-hee's singular concern for Ye-seo, urges her to flee. Yet, Jae-hee's resolve to evade has waned. Dong-woo, at his mental limit, receives a breakthrough when Jang-ho discovers a livestream heralding Jenny's triumphant return. Recognizing the backdrop as Hangil Cleaning Service, Dong-woo hastily makes tracks towards it. Simultaneously, Jung-kwon persuades Gil-ho to abandon their old life and allow Jae-hee to embark on a fresh journey.
Gil-ho decides to depart, contingent upon Dong-woo's assurance of Jae-hee's safety. However, the frightened cop fails to answer his phone, adding to the tension. The ultimate catalyst arrives in the form of a voice message from Su-in's phone, propelling Gil-ho towards the Hangil building with urgency. The message, from Jae-hee herself, conveys her intention to accompany Jung-hwa, leaving Gil-ho with a heart-wrenching decision to confront.
Dong-woo arrives at the building ahead of everyone, yet his journey concludes abruptly in the security room, where a CCTV screen displays Jae-hee's quarters. Unexpectedly, K strikes with swift precision, injecting Dong-woo with a sedative. As Dong-woo collapses, Mr. Choi emerges, revealing that Jae-hee has been exchanged for Ye-seo in a deal gone awry.
Switching to the livestream, Jae-hee defiantly shatters a window and perches precariously on the sill, cursing Gangnam Trading and asserting her autonomy over her own fate. Su-in weeps openly as Jae-hee takes the plunge. Gil-ho arrives moments too late, discovering Jae-hee's lifeless body atop a vehicle. His gaze falls upon K and Choi speeding away, prompting him to chase after them, but a sudden collision with a car halts his pursuit.
Ye-seo, too, receives a haunting voicemail from Jae-hee, acknowledging Ye-seo's strength and acknowledging her as the true savior of Jae-hee's life. A poignant flashback unfolds, depicting Jae-hee poised to leap from a school rooftop until she hears bullies and, in a moment of bravery, descends to shield Ye-seo.
At Jae-hee's funeral, only the Kang family is present, a scene marred by Ye-seo's accusation that Dong-woo is Jae-hee's killer before storming off in distress. Gil-ho stages a solitary memorial, burning Jae-hee's possessions, including cherished photos featuring Jung-hwa and Ye-seo. He then contacts Su-in, issuing a chilling warning to K that he is hot on their trail.
As Episode 5 draws to a close, Prosecutor Min finds himself demoted, while Mrs. Ryu surreptitiously deposits evidence pertaining to Joon-seo's case. She learns that Choi Hak-ju is none other than the CEO of Hae-il Tech. Utilizing Joon-seo's phone, she orchestrates a confrontation, threatening to expose both Choi and Prosecutor Tak. Choi, intrigued by her daring, dismisses her threats but invites her to join his ranks, recognizing her potential.
