Episode 7 of Nine Puzzles unfolds with I-na uncovering a startling revelation: Yoon-su possesses thirteen apartments within The One City complex, twelve of which cater to his family's needs. The solitary exception, Unit 5802, was transferred by Yoon-su to Chi-mok mere months after its acquisition, and a subsequent month witnessed Chi-mok bestowing it upon his then-fiancée, Mi-young. Remarkably, this apartment has been devoid of any reported incidents.
I-na and Han-saem conjecture that Unit 5802 served as compensation for a crime purportedly committed by Yoon-su, Chi-mok, and Mi-young, with Dong-hoon suspected of covering up their misdeeds. However, the enigma deepens: what inducement could Dong-hoon have received for his silence?
Hints scattered across the puzzle pieces offer glimpses into their alleged criminal acts – Dong-hoon discharging a firearm at a doll, Mi-young whispering secrets to a young girl, Chi-mok donning an armband and frolicking with women, and Yoon-su demolishing a sandcastle in frustration. Amidst a standstill in their investigation, Han-saem and I-na's tensions escalate into a brief spat before they retreat for coffee. Seeking insight, Han-saem inquiries about Dong-hoon's character from Dong-su, who, though cautious, praises Dong-hoon's exceptional qualities. Dong-hoon, he reveals, was unfairly scapegoated in the Jungbu serial murder case to appease the public's thirst for justice.
Elsewhere, I-na attends her therapy session, where Dr. Lee expresses curiosity about her roommate but finds I-na's desire to discuss her uncle more compelling. I-na acknowledges the complexity of being a good person to all, confessing that she chose to bury the memory of her uncle's murder due to a deep-seated fear of what she might uncover. Dr. Lee, who empathizes, draws a boundary when I-na probes further into her own therapeutic journey.
As I-na makes her exit, she unexpectedly encounters Sergeant Nam, who, weary of the omnipresent secrets, unleashes his guilt over Mi-young's tragic demise. In a rather insensitive tone, I-na remarks that Mi-young's disregard for the red light might have been a desperate cry for assistance. However, Nam realizes with a chill that the perpetrator could have ended their lives too if he had decided to pull Mi-young over instead.
Turning back to the enigmatic case, Han-saem and I-na delve into her uncle's files. Han-saem cautions her about the lurking specter of corruption and ponders whether she can objectively profile her uncle without any prejudice. Meanwhile, Dong-su alerts Chief Hyun to Han-saem's sudden and intense curiosity regarding Dong-hoon. Both men, however, grapple with the mystery surrounding their friend's death and its elusive motive.
Han-saem finally stumbles upon a pivotal clue—the Myungsung Daily defamation suit. The plaintiff in this case is none other than the brother of the last victim in the Jungbu saga. Intriguingly, Dong-hoon dismissed the defamation suit merely three months after it was filed. As the duo sets out to confront the plaintiff, Han-saem confides in I-na about the missing handkerchief and expresses his unwavering trust in Captain Yang's innocence. I-na, however, wonders if Han-saem will be able to set aside his bias should Yang's actions prove otherwise.
The plaintiff unveils a stunning revelation: Lee Gang-hyun, a reporter from Myungsung Daily, accused his sister of being an escort without any concrete evidence. Tragically, justice seemed to elude him until the reporter took his own life, leaving the duo astonishment-ridden. Delving deeper into Reporter Lee's case file, they uncover a series of phone calls with Oh Cheol-jin, a name that rings a bell for I-na. Subsequently, they stumble upon a crucial clue—Lee had passed away eight months prior to Dong-hoon, hinting that he might have been the Puzzle Killer's inaugural victim.
Han-saem promptly updates Captain Yang on these startling discoveries. However, Yang cautions Han-saem against placing his trust in I-na. Hyun, equally perplexed, seeks an explanation for Han-saem's fascination with old cases but relents when Han-saem vows to present evidence.
Meanwhile, I-na meticulously scours through the evidence, while Han-saem grills the lead detective. He is astonishment-struck to learn that Yang was previously involved in the case before his transfer. On the other hand, I-na unearths the haunting puzzle—a figure clutching a camera, its eyes sewn shut in a macabre display.
Captain Yang attempts to interrogate Choi San, who discovers a functional CCTV camera behind Griffin bar. Yang inquires about the missing handkerchief, but San's responses are evasive and vague. Their conversation is abruptly halted by Han-saem, who confronts Yang in private. Han-saem, who became a cop due to his fascination with mysteries,admired Yang for his unwavering belief in justice. However, he is now distressed and bluntly accuses Yang of being the Puzzle Killer's accomplice.
Yang clarifies that he remained tight-lipped about Lee Gang-hyun's case due to his mistrust of I-na. On the fatal night, Gang-hyun's son, Seon-wook, had a math study session, where I-na was among the attendees. Gang-hyun's demise occurred sometime between 10 PM and midnight. Intriguingly, I-na ventured to the rooftop at 11 PM and was captured by CCTV leaving at 12:23 AM, leaving her without a credible alibi. Dong-hoon eventually transferred Yang after his relentless probing into the matter.
Through I-na's retrospective lens, we witness her arrival amidst Gang-hyun's incoherent ramblings as he, intoxicated, pleads for Cheol-jin's assistance. At the stroke of eleven, overwhelmed by a suffocating sensation, she retreats to the rooftop. Moments later, her gaze catches a mysterious figure entering the premises.
As anticipated, Han-saem confronts I-na, demanding to know why she kept silent about her connection to Reporter Lee. She counters by reminding him that Yang, likewise, remained tight-lipped about his own ties until Han-saem confronted him directly. She presents him with an ultimatum: trust her or trust Yang; a choice he must make.
The following day, having realized Cheol-jin's friendship with Dong-hoon, I-na embarks on a journey to Jeju to visit him. When inquiries about Gang-hyun's wellbeing arise, Cheol-jin's amiable facade transforms into a menacing demeanor.
Deep in contemplation, Han-saem pays I-na a visit, only to be astounded by the sight of her being apprehended as a murder suspect.
The seventh episode of Nine Puzzles culminates in a dramatic twist as a dockyard worker discovers I-na, struggling to pull Cheol-jin's drowned body from the water, painting a haunting tableau of despair and revelation.
