The premiere of "Mary Kills People" kicks off with Choi Kang-yoon, a somber figure, addressing the camera, resolute in his decision to end his life, a serene acceptance etched across his features. Dr. Woo So-jeong, with a glass of champagne subtly laced with a potent chemical, extends it to him. As Kang-yoon's eyelids gently flutter shut, Dr. So-jeong's colleague, Choi Dae-hyun, verifies his departure with a solemn nod.
Just as they prepare for a discreet exit, Kang-yoon's wife unexpectedly returns, sending So-jeong and Dae-hyun into a frenzy of urgent departure preparations. Suddenly, Kang-yoon's body convulses — he is still clinging to life! Forced to see the task through, So-jeong advances towards him with a pillow, her heart heavy. With a final, desperate plea from Kang-yoon for mercy, she reluctantly suffocates him, sealing his fate.
On their return journey, Dae-hyun insists that they had checked for a pulse, finding none. Doubts creep in as they ponder over the efficacy of the drug they administered — benponarbital sodium, known among them as Bavron. While So-jeong heads to the hospital, duty-bound, Dae-hyun finds solace in cooking dinner for So-jeong's niece and nephew, Jae-yeong and Jae-yoon, the warmth of family a brief respite from their grim deeds.
Later that evening, Dae-hyun ventures into a club, seeking out his supplier, Kim Si-hyun. Accusations fly as Dae-hyun charges Si-hyun with diluting the chemical. Si-hyun admits without hesitation, revealing it was a test to ascertain Dae-hyun's loyalty. pulsating rhythm of the club.
At the bustling hospital, So-jeong crosses paths with Ye-na, a junior doctor who doubles as a colleague in her clandestine venture. Ye-na unfolds the tale of Jo Hyun-woo, a gentleman battling a brain tumor and potentially the next addition to their clientele.
Their next encounter brings them to Lee Eun-young, an elderly lady who, following the loss of her husband, has been diagnosed with terminal cancer. Dae-hyun, sensitive to her plight, learns that her family cherished their happiest moments by the seaside. He proposes that she takes her final journey there, amidst the waves that once brought them joy. So-jeong, cautious about the public nature of the location, voices her skepticism. Yet, Dae-hyun skillfully identifies a secluded spot, untouched by prying eyes. Eun-young, eager to reunite with her beloved ones, sips the champagne with a hopeful heart. Silently, she surrenders to the embrace of eternity, prompting So-jeong to swiftly usher them away from the serene yet solemn scene.
The narrative shifts to a different setting where Detective An Tae-sung delves into the mysterious demise of Baek Hyun-seung, a young boy. The boy's father, Baek, a powerful legislator, insists that Tae-sung wrap up the investigation within a month. An underlying awareness of euthanasia practices pervades their conversation, and news of footballer Choi Kang-yoon's passing soon reaches Tae-sung's ears.
Elsewhere, So-jeong and Dae-hyun arrange a consultation with Jo Hyun-woo, a diligent deliveryman seeking their services. They deliberate on So-jeong's role, with her vowing to scrutinize his medical history before committing. Meanwhile, Kang-yoon's wife stumbles upon documents from a Swiss euthanasia provider amidst his possessions, unraveling another layer of his untimely end.
In a different corner, Jae-yeon's tranquil home becomes the stage for a visit from Woo-mi, an old classmate. The air between them thrums with burgeoning romantic sentiments, leading to an invitation for a sleepover. Jae-yeon, in search of her brother's swimming flippers, finds herself aided by Woo-mi. Amidst their search, Woo-mi discovers the Bavron bottles, and, unable to resist its allure, secretly retains one for herself.
At the school event exclusively for parents, So-jeong finds herself in unexpected company: An Tae-sung, whose daughter is revealed to be Jae-yeon's rival. The day takes a peculiar turn when a teacher pulls So-jeong aside to inform her that Jae-yeon has been discovered with condoms in her possession. That evening, So-jeong attempts to reassure Jae-yeon that harboring romantic feelings is perfectly normal, but Jae-yeon fiercely denies any ownership of the condoms.
Later, at Choi Kang-yoon's funeral, a detective approaches his grieving wife with a request for an autopsy. Initially hesitant, she eventually concedes after the detective presents evidence implicating Baek.
Returning home later, So-jeong discovers her children at the neighboring house, where she encounters Woo-mi, the neighbor's daughter, none other than Jae-yeon's classmate. Woo-mi and Jae-yeon plead with So-jeong to allow Jae-yeon to stay over. So-jeong, obliging, agrees. That very night, Woo-mi mocks her mother's naivete and makes advances toward Jae-yeon, revealing a darker side.
The subsequent day, Hyun-woo experiences excruciating pain while riding his motorcycle, leading to a harrowing accident. So-jeong, assigned as his attending physician in the ER, witnesses his persistent agony despite various painkillers. Despite his suffering, Hyun-woo insists on leaving the hospital and wanders into the bustling streets outside. So-jeong swiftly intervenes, rescuing him from impending danger and bringing him back to the hospital, where she ultimately decides to perform euthanasia to end his torment.
Meanwhile, at school, during a mesmerizing ballet performance, Woo-mi abruptly collapses on stage, shocking everyone present. Elsewhere, Detective An and his team scrutinize their suspect list, which notably includes So-jeong, Dae-hyun, and Ye-na.
The narrative concludes with an epilogue flashing back to two years prior. Inside a serene church, So-jeong confesses to a priest about forcing a patient to endure two years of treatment, only for the individual to ultimately perish. She bemoans the additional suffering her actions caused. With a heavy heart, she informs the priest of her decision to abandon her religious beliefs and relinquish the name Mary.
