Episode 7 of "Mary Kills People" kicks off with a poignant flashback scene. As Mi-na's health deteriorates rapidly, Dae-hyun finds himself grappling with the heart-wrenching decision to grant her euthanasia. Meanwhile, So-jeong arrives unexpectedly at his house and catches a glimpse of him holding a vial of Bafron. Urging him to take Mi-na to the hospital, So-jeong encounters fierce resistance from Mi-na herself, who confides that every passing moment feels like an eternity of torment. Conceding to Mi-na's wishes, they embark on a poignant journey to her childhood neighborhood, where So-jeong administers the drug-laced drink, marking the beginning of Mi-na's peaceful end.
In the present timeline, Dae-hyun steels himself to confess to Mi-na's passing, only to be halted by Yi-soo, who reveals a stunning revelation: his family has bribed her to ensure his freedom, especially amidst the joyous occasion of his sister's impending wedding.
Elsewhere, Ji-hoon informs So-jeong that she is free to leave since Father Yang has confessed to her attempted murder. Unwavering in her quest for truth, So-jeong insists on staying put until the autopsy results are in. When the results are finally unveiled, they leave every detective baffled—there's no trace of Bafron mentioned.
Returning to her duties, So-jeong's world is soon shaken once more when Ye-na's identical twin brother, Geon-soo, collapses on stage during a band performance and is rushed to the ER. In a poignant encounter, Geon-soo confides in So-jeong that he has been battling cystic fibrosis for years and has been given a mere three months to live, a truth hidden from Ye-na. Aware of So-jeong and Ye-na's secretive line of work, Geon-soo pleads for her to "treat" him, though he knows Ye-na would vehemently oppose it. Agreeing to help, So-jeong resolves to tell Ye-na the truth first, but Geon-soo understands the challenges that lie ahead.
Detective Bu, meanwhile, receives a tip about a substantial drug shipment entering the country under Kim Si-hyun's name. However, the drug squad swiftly dismisses it as a fake lead. Undeterred, Detective Bu reaches out to Yi-soo, imploring her to persuade Dae-hyun to lend them his expertise once more, amidst a web of deceit and desperation.
Dae-hyun ventures to the welfare hospital, where fate brings him face to face with Song-yi. With a compassionate heart, he whisks her away to purchase fresh attire, transforming her appearance with new clothes. His tranquil afternoon is interrupted by a call from Yi-soo, prompting him to reconvene with her, Song-yi in his companionable care. Yi-soo, capitalizing on his empathy for the child, manipulates the situation, urging him to orchestrate So-jeong's arrest.
Elsewhere, So-jeong encounters Ye-na at the welfare hospital, unburdening her soul about her brother's precarious state and his heart-wrenching plea for euthanasia. Ye-na, her emotions in turmoil, implores So-jeong with all her might to save Geon-soo from his fate. Later, So-jeong seeks forgiveness from Father Yang, her voice tinged with contrition. However, Father Yang, his face etched with disappointment, rejects her and warns her to stay away, insisting that mortal hands should not play God. He threatens to expose her should she cross this boundary again. So-jeong, unyielding, labels him irresponsible, reminding him of his refusal to aid her desperate mother.
At the police station, Ji-hoon persuades Detective Bu to defy the drug squad's declaration of fake news and head to the alleged drug exchange site. Dae-hyun, returning to the hospital with Song-yi, crosses paths with So-jeong, who announces her intention to temporarily take the little girl home. Within the hospital walls, Ye-na confronts her brother, Geon-soo, his words yearning for serenity met with her furious denial.
So-jeong and Song-yi journey to the columbarium, paying their respects to Lee Eun-hee's eternal rest. There, So-jeong solemnly vows to fulfill her mother's dying wish by caring for Song-yi, intent on taking her home. Unbeknownst to them, Ji-hoon's unexpected arrival at the columbarium catches snippets of their conversation. He cautions So-jeong that social services will soon come knocking, despite her temporary refuge plan.
Elsewhere, Gwang-chul reaches out to his sister, inquiring about an acquaintance who had planned to seek euthanasia in Switzerland but couldn't due to deteriorating health. He reveals their offer to provide a similar service.
Meanwhile, Ye-na's mind drifts back to a tumultuous childhood, where their mother was a beacon of protection against their abusive father. After her passing, Geon-soo became Ye-na's shield. Now, amidst tears, Geon-soo envelops Ye-na in a comforting embrace, her voice cracking as she admits her helplessness at the thought of life without him.
At the stakeout, Ji-hoon and his fellow officers witness a clandestine gathering as a gang prepares to exchange a consignment of illicit drugs. Confronting the suspects head-on, they plunge into a heart-pounding car chase, but despite their best efforts, the drug traffickers slip through their fingers.
Elsewhere, Ye-na encounters So-jeong and entrusts her with taking on Geon-soo's case. Doubts cloud Ye-na's mind as she ponders the wisdom of her decision, prompting So-jeong to confide in her about her mother's passing. Initially consumed by anger, So-jeong reveals her eventual realization of her mother's unwavering love. She emphasizes that the utmost we can do for our beloved is to assure them of our wellbeing, so they rest easy in their absence.
Meanwhile, Ji-hoon receives a crucial tip about the last known location of Kim Si-hyun's vehicle. Rushing to a nearby lake, tensions mount as the submerged car of Kim Si-hyun is laboriously pulled from the depths of the water, adding a grim new chapter to their unfolding investigation.
