The premiere episode of "Gangnam B-Side" kicks off with an electrifying drug raid at a sleek Gangnam club. Captain Kang Dong-woo and his crack team infiltrate the premises, defying the Chief's attempts to halt their bold move. As the authorities sweep in and apprehend all suspects, the Chief attempts a clever maneuver to isolate Dong-woo, but the captain, senses something amiss and bolts towards the basement. There, he stumbles upon Detective Jang-ho, who is busy stuffing a sack with illicit narcotics.
During the interrogation session, Jang-ho plays the role of the scapegoat, claiming he's merely a courier in a web of corruption that entwines almost everyone in Major Crimes. Shortly thereafter, the Chief finds himself under arrest and issues a chilling threat to bring Dong-woo down with him.
Three years fly by, and the underbelly of Gangnam remains active. A minor drug pusher named Lemon peddles his wares to VIPs in another club's dimly lit corners. Among the escorts, Jenny catches a glimpse of a menacing text from aJjuni Boy, warning a VIP of dire consequences if she's lost. Panicked and suddenly lucid, Jenny dashes to the toilets, retrieves a hidden phone, and flees for her life.
Elsewhere, Jung-hwa, another escort, faces brutality from a ruthless thug who discovers she doesn't have the incriminating video they seek—a video that Jenny possesses. He alerts the club's bouncers, and a frantic hunt for Jenny begins. Meanwhile, at a neighboring establishment, Yoon Gil-ho comes to the rescue of a drugged escort, doling out justice to her dealer with a punch-laden warning never to prey on his girls again. Jenny, who is under Gil-ho's protection, ignores his calls as she desperately tries to reach Jung-hwa, hidden away in an old, abandoned arcade.
Despite her silence, Gil-ho tracks Jenny down, his suspicion growing that she might be tangled with the enigmatic Noh Joon-seo. Jenny, fearful for Jung-hwa's safety, stays tight-lipped. Gil-ho vows to locate Jung- hwa, soothing Jenny's frayed nerves with his promise of safety.
Regarding Dong-woo, he has ventured into the countryside, entangled in his latest case—a DUI incident involving the mayor's nephew. When the young offender discovered narcotics in his vehicle, he attempted to corrupt Dong-woo with bribery. However, the resilient cop swiftly counterattacked, subduing and arresting him. Just then, Commissioner Moon appeared on the scene, revealing himself as Dong-woo's guiding mentor.
Moon's arrival was timely, as he disclosed that several individuals in Gangnam had gone missing, Jenny alias Jae-hee among them. He earnestly tasked Dong-woo with solving the mystery. Dong-woo hesitated, revealing the reason behind his self-imposed isolation—a shattered trust in fellow officers stemming from a botched drug raid. Moon, sensing Dong-woo's potential, urged him not to let his talents wither and handed him the case file.
As Dong-woo delved into the case, he stumbled upon social media videos from club-goers, depicting Jenny fleeing from bouncers. A sudden call from the hospital interrupted his investigation, urging him to rush there. His daughter, Kang Ye-seo, was under suicide watch, mentally frail due to relentless bullying at school. Dong-woo's heart raced as he reached her bedside, noticing a needle mark and an intricate tattoo of a crying woman on her wrist.
His estranged wife soon joined him, accusing him of never being there to support his daughter. As a busy news anchor, she also failed to give Ye-seo enough companionship. She blames Dong-woo for Ye-seo's bullying and an accidental death on a rooftop.
The following day, Ye-seo texted Dong-woo to pick her up after being discharged. He made an effort to reconnect, though she remained distant. She solicited his help in finding Jae-hee, her cherished friend. Dong-woo, initially dismissive, provoked her ire for being judgmental and feigning concern. Curious about her insistence, she confessed that she couldn't imagine life without Jae-hee. She forwarded a photo, revealing that Jae-hee bore the same wrist tattoo.
Flashing back to a year ago, Jae-hee is depicted in a dire need of assistance. Ye-seo dials Dong-woo's number, yet the encounter sours almost instantaneously. Dong-woo castigates Jae-hee for her delinquency, prompting Ye-seo to accuse him of being an absent father. His temper flares, convinced that she holds some culpability for a tragic rooftop accident.
In the present, Dong-woo reconnects with an old associate, Ji-su, and inquiries about Jae-hee. The young woman's sole offense is a charge of larceny. Ji-su's eyes widen as he recognizes Jae-hee's tattoo from a clip circulated by an underground spycam ring. A similar tattoo adorns a girl slumped unconscious in a club's lavatory. Gil-ho scoops her up and deposits her into a car alongside Jae-hee. The police spot him, their memories jogged by an encounter three years prior during a drug bust, where a customer brutalized an escort, necessitating hospitalization. Gil-ho, enraged, retaliates with violence, leading to his arrest. Dong-woo, however, releases him, merely a chauffeur for pimps, cautioning him to steer clear of his radar.
Currently, Ji-su faces a hurdle: he cannot trace Gil-ho's car plates due to a prosecution embargo. They cite an ongoing case and introduce a mysterious prosecutor. Prosecutor Min Seo-jin descends upon Gangnam, declining all offers of assistance, steadfast in her resolve to solo the Gangnam Missing Persons case.
That night, Gil-ho lays a trap for Lemon, subjecting him to brutal torment until he learns that Jung-hwa has been whisked away to an "event." Jung-hwa is escorted by a thug to a hotel suite filled with VIPs. The arrival of a man donning white snake leather shoes strikes terror into her heart. As Gangnam B-Side Episode 1 concludes, Gil-ho discovers Jung-hwa lifeless, the room deserted, leaving a haunting tableau.
