Episode 6 of "Mary Kills People" kicks off with a poignant scene where So-jeong administers the euthanasia drug to Lee Eun-hee, her eyes filled with sorrow as she watches her gently slip away. Upon Eun-hee's passing, Ji-hoon wave swiftly on the scene, accusing So-jeong of murder and hauling her off to the precinct.
At the police station, Ji-hoon grilled So-jeong about the circumstances surrounding her mother's death, accusing her of callously tying a cloth around her mother's wrists before she plunged into the lake. So-jeong, tears welling up, protested that she had merely misunderstood her mother's intentions, thinking she was about to engage in a playful activity with some local children. Ji-hoon remained unconvinced.
Father Yoon, stepping in to offer clarity, relayed to Ji-hoon that So-jeong's actions were driven by a desperate attempt to prevent her mother from taking her own life. The narrative then shifts to a poignant flashback, revealing a distraught 16-year-old So-jeong rushing to Father Yoon for help after she couldn't find her mother in the lake. Returning to the present, Father Yoon confirmed to Detective Bu that Lee Eun-hee had succumbed to her illness, a revelation that left Councilman Baek deeply frustrated.
Meanwhile, Gwang-chul confronted Tae-sung, demanding his intervention to secure So-jeong's release. Ji-hoon delved deeper into the case, uncovering a two-year-old incident involving a woman named Kwon Kyung-mi and her terminally ill daughter, who had reportedly died as a result of assisted suicide. Detective Bu confronted So-jeong about this connection, but she invoked her right to silence.
The detectives scattered, interrogating all of So-jeong's known associates. Detective Bu grilled Dae-hyun, accusation flying as he suggested that Dae-hyun might have been responsible for the death of his terminally ill ex-girlfriend, Mi-na.
A flashback transported us back two years, showing Dae-hyun arriving at So-jeong's house for lunch. So-jeong, aware of his past struggles with drug addiction, warned him to keep her informed if he relapsed. Later, at the hospital, So-jeong assisted a man with a leg injury, her professionalism unwavering.
The scene then shifts to a heart-wrenching moment when Kwon Kyung-mi brought her daughter, suffering from stage 4 cancer and in excruciating pain, to the emergency room. Kyung-mi, tears streaming down her cheeks, pleaded with So-jeong to end her daughter's suffering, begging her to grant her child a merciful death. So-jeong, her eyes reflecting the gravity of the request, refused.
Later on, the man from earlier lends a helping hand to So-jeong with some much-needed house repairs. Upon returning to the hospital, she is taken aback to discover that Kyung-mi, the mother, has been apprehended for murdering her own daughter. Elsewhere, the injured man eventually finds new employment at a bustling construction site.
Some time later, two burn victims are rushed to the emergency room. Tragically, one passes away, while the other, who suffered a leg injury, finds himself in excruciating pain, pleading with So-jeong to end his suffering. She gently urges him to be patient. Repeatedly, he begs the doctors for mercy, yet none comply. So-jeong, moved by his plight, even takes it upon herself to cover the costs of his treatment.
At Choi Dae-hyun's residence, an unexpected visitor arrives in the form of his former lover, Mi-na. She reveals her terminal illness and humbly requests his assistance in ending her life peacefully.
In the present, Tae-sung contacts Gwang-chul, vowing to ensure that no traces of Bafron are detected in the autopsy report. Meanwhile, Yi-soo bursts into the police station, accusing the detectives of illegally detaining Dae-hyun. As they prepare to depart, Dae-hyun confesses to Yi-soo his desire to surrender himself.
Flashing back to earlier events, Kyung-mi, unrepentant, tells reporters that she harbors no regrets, as her daughter is now free from pain. The peaceful hospital atmosphere is shattered when the burn victim, driven to desperation, threatens a doctor with a knife. So-jeong rushes to the scene, where the distressed man implores her to let him leave the hospital. Despite her best efforts to persuade him to fight for his life, he insists he is living in a personal hell and takes his own life with the knife.
In the present, Father Yoon expresses astonishment at So-jeong's involvement not only in Lee Eun-hee's death but in other fatalities as well. Suddenly, a stunning confession emerges from Father Yoon, who admits to killing Lee Eun-hee. Simultaneously, Dae-hyun confesses to Yi-soo that he is responsible for everything, starting with the murder of Mi-na.
Another memory resurfaces, showing Gwang-chul visiting Father Yoon's hospital while Lee Eun-hee was still alive. He hands So-jeong a vial of Bafron, and she injects some into her respirator. After Eun-hee passes away, Gwang-chul departs with the syringe and the remaining Bafron.
In a final memory, So-jeong's mother's demise is revisited. This time, the scene unfolds with greater clarity: So-jeong desperately leaps into the lake, attempting to untie her mother's bonds. However, her mother, resolute, refuses her daughter's assistance.
