Episode 3 of "Mary Kills People" kicks off with a bombshell revelation: Cha Hyun-woo is actually Police Detective Ban Ji-hoon in disguise! A nostalgic glimpse into the past, three months earlier, depicts Detective Bu encountering Ji-hoon, then a humble traffic cop, and summoning him back to his original unit. Bu unveils a daring undercover operation, introducing Ji-hoon to a terminally ill man named Cha Hyun-woo, who is battling a brain tumor. This is how Ji-hoon transforms into Cha Hyun-woo, embarking on a deceptive journey.
A swift montage unravels, exposing that everything witnessed thus far has been a meticulously crafted lie, with Ji-hoon subtly ensnaring So-jeong in a trap. Hidden cameras dot the apartment where Ji-hoon falsely confessed his desire to die, revealing a sinister plot.
Elsewhere, Woo-mi's mother, Yi-soo, discovers her daughter has consumed a drug retrieved from So-jeong's abode. She confronts So-jeong, vowing justice for her daughter's plight.
Meanwhile, Dae-hyun receives an update from Soo-young's father, who reveals he reached out to Soo-young's estranged mother but was rejected. He implores Dae-hyun to keep this from Soo-young. During their meeting, Dae-hyun diverts Soo-young's attention by arranging a meeting with his crush, Hye-jeong, even giving him a mini makeover to boost his confidence.
At the hospital, So-jeong crosses paths with Choi Kang-yoon's wife, who wishes to thank all the doctors who aided her husband. She excuses herself to fetch So-jeong's gift from her car, leading them to the parking lot. In private, she expresses gratitude to So-jeong for her assistance and confesses her inability to let go of her husband. She hands over euthanasia papers and an unlisted phone found among her husband's possessions.
Simultaneously, Detective Bu realizes that the genuine Cha Hyun-woo has vanished from the hospital where he was confined. With Dae-hyun's assistance, Soo-young successfully meets Hye-jeong and asks her out, to which she agrees for the following day. That evening, So-jeong informs Dae-hyun of their dwindling Bavron supply—just one bottle remains, the new shipment having been shattered in the hospital fracas from the previous episode.
The date between Soo-young and Hye-jeong blossoms beautifully, culminating in a tender kiss on his cheek from Hye-jeong. Yet, their evening must abruptly conclude when Soo-young is racks by a coughing spell. Hye-jeong insists he pledges to meet her again the following day.
Meanwhile, So-jeong arrives at the house meticulously prepared by Ji-hoon. She is in the midst of unpacking her belongings when Father Yang calls with devastating news: the true Hyun-woo has been discovered deceased. Sensing something amiss, So-jeong feigns forgetfulness, claiming she left something in her car, and hastily exits the premises. Outside, she shatters a bottle of Bavron on the pavement and speeds away, her departure alerting Ji-hoon and his associates that their charade has been uncovered.
She promptly contacts Dae-hyun, who informs her that Soo-young's condition has taken a dire turn and he urgently requires the medication. Determined, So-jeong personally confronts Gu Gwang-chul and persuades him to sell her a bottle of the drug.
With time of the essence, she rushes to Soo-young's home, where father and son share a poignant embrace, exchanging heartfelt "I love you"s. Soo-young entrusts his father with the task of meeting Hye-jeong at the park the next day to break the news to her gently. He then accepts the potion from So-jeong and passes peacefully away.
Some time later, Dae-hyun returns Soo-young's phone to his grief-stricken father. Upon reviewing its contents, he discovers a video message from Soo-young, in which he confesses his awareness of the truth regarding his mother. Yet, he reassuringly states that he harbors no ill will because he has an exceptional father. Keeping his promise, Soo-young's father ventures to the park to meet Hye-jeong and impart the heart-wrenching news.
