The narrative of Mary Kills People's fourth episode unfolds with an introductory dialogue between So-jeong and Dae-hyun. His inquisitive gaze falls upon her, asking if her encounter with Gu Gwang-chul had passed smoothly. She nods, her expression serene, but a haunting flashback ripples through the scene. In this memory, Gwang-chul demands she disrobe, capturing her image with a cold precision. He confesses his awareness of their underhand dealings with his narcotics but nonetheless hands her the vial of Bavron, his tone laden with menace.
Meanwhile, in the present, Ji-hoon confronts So-jeong at the hospital, his eyes burning with resolve. He warns her to desist from her clandestine activities, vowing to apprehend her. She stands her ground, insisting her actions are devoid of criminality. Suddenly, headlines splash across news channels, announcing Kim Si-hyeon's arrest in the midst of a drug transaction. However, Ji-hoon's sharp instincts reveal the apprehended individual is but a decoy, not the real Kim Si-hyeon.
Amidst this turmoil, So-jeong instructs Ye-na to temporarily halt their services at the hospital. Councilman Baek, enraged by this sudden halt, issues a stern command to Detective An. He mandates a search warrant for the hospital, urging An to use any pretext to uncover evidence against So-jeong.
Elsewhere, Ji-hoon and Detective Bu deliberate on their next course of action when they are interrupted by the startling news: the search warrant for the hospital has been granted. With urgency, their team descends upon the hospital, casting a shadow of dread over So-jeong. Fortunately, she has already disposed of incriminating papers, while Dae-hyun and Ye-na have sought refuge at Father Yang's welfare hospital.
Navigating the corridors of the hospital, So-jeong encounters Yun-hee, a patient battling both diabetes and terminal cancer. Despite her dire medical needs, Yun-hee refuses further tests, citing financial constraints. She yearns to preserve every penny for her daughter's future. Moved by her plight, So-jeong allows her to depart but offers a beacon of hope—Father Yang's hospital's address, where treatment awaits her free of charge.
Elsewhere still, Gu Gwang-chul's anger simmers as he discovers Kim Si-hyun's betrayal, continuing to sell to Dae-hyun despite explicit instructions to the contrary. This revelation stems from an elderly spy within his ranks, who has been keeping tabs on Si-hyun. Gwang-chul's mind is made up; Si-hyun's days are numbered.
The detectives' investigation homed in on a luxurious penthouse nestled in Gangnam, where they crossed paths with a former drug dealer who freely admitted that someone of great significance resided there. In parallel, So-jeong attended a gathering with fellow parents from Jae-yeong's school, among them Yi-soo and Detective An. The conversation inevitably steered towards Woo-mi's fainting episode during the ballet performance, prompting So-jeong to firmly assure everyone that Woo-mi's condition was solely due to malnutrition, categorically denying any involvement with drugs.
Elsewhere, Kim Si-hyun found himself being steered down a dimly lit alley by the elderly henchman of Gu Gwang-chul. Moments later, the air was shattered by the sound of gunshots. Meanwhile, Yun-hee found herself at Father Yang's hospital, where Dae-hyun administered medication, yet her agony persisted unabated.
At So-jeong's hospital, her shock was palpable when she stumbled upon Kim Si-hyun, now going by the alias Kim Jae-hoon, lying wounded in the emergency ward, the victim of gunshot injuries. Inevitably, the police were summoned. Elsewhere, Gu Gwang-chul fumed over his assassin's haphazard work. He rang the hospital, reaching So-jeong and instructing her to stall while he rectified the situation.
Ji-hoon's realization dawned: the penthouse owner, Kim Jae-hoon, was indeed the genuine Kim Si-hyun, recognizable by his distinctive tattoos. He hurried to the hospital and confirmed his suspicions. Later, So-jeong transferred Si-hyun to the ICU.
One of the detectives then contacted Detective An, imparting fresh intelligence that implicated a police officer in league with Kim Si-hyun. As Detective An stepped into his vehicle, he was ambushed by Gwang-chul, who pointed a gun at him, revealing Detective An as the corrupt cop. Gwang-chul leveraged this information, compelling him to remove Si-hyun from the hospital, but An protested it was nearly impossible.
Gwang-chul subsequently confronted So-jeong, revealing his awareness of her using his drugs for euthanasia. He demanded she remove Si-hyun, but she steadfastly refused to succumb to blackmail. The following day, two men disguised as maintenance workers infiltrated the hospital, orchestrating minor explosions in the cafeteria—all at Gwang-chul's bidding. He called So-jeong, threatening further destruction unless she complied. Pleading with him to halt his actions, she reluctantly agreed to spirit Si-hyun away herself.
She makes her way to the Intensive Care Unit and covertly hands him a pair of scissors, whispering urgently for him to take her hostage. Si-hyun, complying without hesitation, manages to escort both of them towards the hospital's secluded rear exit, where Gwang-chul is patiently stationed with his vehicle at the ready. As they tear off down the road at breakneck speed, So-jeong catches a glimpse of Ji-hoon hot on their trail, chasing after the fleeing car with relentless determination.
