Episode 9 of "Mary Kills People" kicks off with a tense scene as Jin Yeong-su's lawyer informs Gu Gwang-chul that he can easily secure more clients willing to pay handsomely for his illicit services. Shortly after, Gwang-chul is hit with another shock when he discovers that Dae-hyun has pilfered the remaining stash of Bafron.
Meanwhile, chaos unfolds elsewhere as So-jeong frantically calls for an ambulance following Gun-soo's sudden demise. Ji-hoon arrives at the scene, his suspicion of So-jeong palpable as he casts a knowing glance in her direction. Dae-hyun, on his way home, finds himself ensnared in a perilous situation when Gwang-chul's henchmen surround his vehicle, causing it to flip over in a dramatic crash.
Rushing to Dae-hyun's side in the ICU, So-jeong confronts Gwang-chul with a fierce glare, accusing him of the heinous act. Gwang-chul, ever the manipulator, attempts to coax her into a partnership, but So-jeong stands her ground, threatening to expose him to the authorities if he dares to harm those she cares about.
Back at the police station, Ji-hoon and Detective Bu embark on a rigorous interrogation of the attendees at Gun-soo's funeral gathering. They meticulously comb through CCTV footage, their minds racing with questions about the origin of the Bafron that ended Gun-soo's life.
In the hospital, Dae-hyun slowly regains consciousness, his eyes fluttering open to find So-jeong by his side. She doesn't hold back, rebuking him for his reckless behavior. Dae-hyun, weary and disillusioned, suggests they abandon their dangerous venture once and for all, as the situation has escalated far beyond their control.
Ye-na arrives at the hospital, her presence a comforting balm as she takes charge of Dae-hyun's care. Amidst the hustle and bustle, So-jeong is called away to attend to a new patient in the emergency ward—a woman who has overdosed on sleeping pills, accompanied by her seemingly concerned husband, Park Tae-hyung.
It turns out that Tae-hyung is one of the clients who sought out Ye-na for her euthanasia services. He reveals that he's suffering from a tumor as well as a form of dementia that triggers hallucinations, causing him to constantly perceive strangers lurking around him who don't actually exist. He pleads with So-jeong to release him from this torment. The detectives meticulously retrace Dae-hyun's movements and come to the conclusion that he might have administered euthanasia to Jin Yeong-su. Meanwhile, Dae-hyun, fearing that Gwang-chul's henchmen will come after him again, requests Ye-na to arrange for his transfer to Father Yang's hospital.
As Tae-hyung's wife recuperates in the hospital, So-jeong discerns that she's battling depression and may be contemplating suicide following her husband's demise. So-jeong insists she can't stand by and let mentally healthy people die, but Tae-hyung's wife contends that her depression renders her unwell.
The detectives then grill Dae-hyun about Jin Yeong-su at Father Yang's hospital. When that line of questioning proves fruitless, they probe him and Ye-na about why they're inseparable, prompting Dae-hyun to fabricate a story about them being romantically involved.
After the detectives depart, Father Yang inquires of Dae-hyun whether he's been engaging in the same practices as So-jeong. He reiterates his belief that such actions are sinful.
Ji-hoon then pays a visit to Gwang-chul and interrogates him, disregarding his superiors' directives. Gwang-chul admits to knowing Councilman Baek, who also introduced him to An Tae-sung. Upon returning to the police station, An Tae-sung reprimands Ji-hoon for his insubordination.
The next time So-jeong encounters Dae-hyun, he informs her that he wants to terminate their euthanasia practice for good. He asserts that their deeds are tantamount to murder. However, So-jeong remains resolute in her stance, and the two part ways on a sour note.
Some time later, Gu Gwang-chul arrives at Father Yang's hospital with hefty donations, sending Dae-hyun into a state of trepidation.
It's revealed that Park Tae-hyung's nephew is none other than Jin Yeong-su's lawyer, who is eagerly awaiting his uncle's demise to claim his inheritance. A flashback unveils that Gwang-chul instructed him to approach So-jeong as a regular client and make a heartfelt plea.
At home, Park Tae-hyung stumbles upon a harrowing scene: his nephew is physically abusing his wife. With a steely resolve, he demands that his nephew leave them in peace and, in a desperate bid to pacify the situation, promises to pen a suicide note exactly as the nephew demands.
Later, news reaches So-jeong through both the nephew and Park Tae-hyung's wife that Tae-hyung's condition has taken a drastic turn for the worse. When So-jeong laments that she doesn't possess any Bafron, the nephew, with a sly glint in his eye, mentions that he has connections on the dark web who can procure it for them.
Meanwhile, Dae-hyun, wracked with guilt, reaches out to Yi-soo and confesses his desire to turn himself in to the police. He even goes so far as to agree to wear a wire, hoping to ensnare So-jeong in her own web of deceit.
In a sudden twist, Dae-hyun then contacts So-jeong, changing his tune entirely. He claims he's had a change of heart and wants to be present for Park Tae-hyung's final moments. They arrange to meet at the apartment, where Dae-hyun arrives, his pockets stuffed with secret cameras and bugs, ready to gather incriminating evidence.
But just as he's about to spring his trap, a call from Gu Gwang-chul shatters the tense silence. In a dramatic gesture, Dae-hyun unbuttons his shirt, revealing the hidden bugs to a stunned So-jeong, whose face contorts in shock.
