Episode 6 of Nine Puzzles kicks off with Han-saem being taken aback to witness I-na setting up their operational headquarters at his abode. A nostalgic flashback unfolds, capturing their moments of camaraderie over drinks, delving into suspicions of a spy within their ranks. In the present, Han-saem relents as I-na presents him with a steaming cup of coffee, her inquisitive eyes landing on his tattoo. He unveils its origin, a mark of his undercover mission as a brute. She deems it "cool," earning a smug grin from him.
Their latest clue in the intricate case of the puzzle killer leads them to Do Yoon-su, the president of DH Construction. Chi-mok's three calls to him on that fateful night have left a trail worth pursuing. Han-saem seeks clarity from Captain Yang, who found it peculiar that Yoon-su claimed ignorance of Chi-mok's existence.
Meanwhile, I-na storms into Yoon-su's office, where he steadfastly maintains his denial of knowing Chi-mok, Mi-young, or Dong-hoon. His rudeness provokes I-na to weave a chilling tale of a taxi driver who met a grim fate by beating a disrespectful passenger to death, whimsically suggesting a needle to the back of the neck for a "cleaner" kill.
Elsewhere, Han-saem probes Yang-hee, who had visited Yoon-su three years prior with Chi-mok, seeking his blessings for their wedding. Instead, Yoon-su was offended. Chi-mok had shared with Yang-hee about cherished memories forged with Yoon-su, memories now tinged with suspicion.
After her therapy session, I-na engages Dr. Lee in conversation about his cohabitation with Dr. Hwang, who dismisses it as merely a cost-saving measure devoid of any romantic undertones. I-na's sharp eye catches a painting by Dr. Hwang, which she teasingly dubs as an imitation, lightening the mood momentarily.
Deeply touched by the safety she feels at Han-saem's place, I-na experiences her first peaceful sleep in a decade. She practically makes herself at home, bringing her belongings, much to Han-saem's amusement tinged with frustration. They pool their findings, concluding that the so-called "good memories" Yoon-su and Chi-mok shared were actually successful scams.
Precisely as expected, Yoon-su meets his fate, felled by an unseen hand. The reporters swarm the building like sharks scenting blood, fueled by whispers of corruption entangling his name. The police officers eye I-na and Han-saem curiously as they arrive side by side, their synchronized presence a mystery. Yang interrogates Han-saem, probing the reason behind his inquiry about Yoon-su, but Han-saem deftly sidesteps the inquiry.
Turning to the grim task of the murder investigation, they discover that the CCTV footage of the crime scene floor has been methodically erased. A needless mark, stark and visible on Yoon-su's neck, suggests a bold provocation from the killer, inviting the authorities to notice. Yang's journalist friend, Reporter Park, hungry for a scoop, speculatively links the murder to the mayor's lobbying scandal and a controversial incident at Yangyang resort. Yang lets him be, their friendship a shield against too-eager probing.
At the hospital, amidst the clinical sterility, I-na and Han-saem receive the fourth puzzle piece—a clown, an enigmatic clue dropped while they await the autopsy results. The medical examiner's verdict: an insulin overdose, the silent killer that ended Yoon-su's life. They ponder how the murderer tracked I-na to the hospital's hallowed halls.
Eager to piece together the puzzle, I-na dashes home alone, her curiosity a beacon in the dark. The room is a canvas of vivid images—a doll pierced by bullets, clouds of cotton candy swirling, children dancing with unbridled joy, a meticulously crafted sandcastle, and a Ferris wheel painting the sky with its rotating lights. I-na wonders if the perpetrators orchestrated some dark ritual or crime at an amusement park, weaving their twisted tale amidst laughter and innocence.
Han-saem reproaches her for her solo dash, and she retorts with playful teasing, sensing his genuine concern. Confident that the killer's game is to lead her to the puzzle's solution, I-na feels a misplaced sense of security.
Yang becomes acutely aware of the evolving relationship between the duo and ponders whether Han-saem might be romantically involved with I-na. Han-saem's blush gives way to a subtle hint, yet it's enough for the captain to discern that the conversation veils a deeper truth tied to Dong-hoon's tragic demise. Their journey leads them to the Yangyang Resort, a sprawling site immersed in construction chaos. Seo Do-ho, the vigilant overseer, unfolds the intricate politics of the resort, highlighting Yoon-su's indispensable role amidst these complexities. When queried about unusual occurrences, he unveils Kang Chi-mok, Yoon-su's golf companion, whose inebriated ramblings often left the usually brash Yoon-su unusually quiet.
Elsewhere, I-na stumbles upon Dream Land, the amusement park featured in the perplexing puzzle. She discovers its impending closure and learns from the operator that it has been free of significant mishaps. Meanwhile, Han-saem returns home to find a trove of photographs depicting the park, adorned with a myriad of balloons. A haunting intuition tells him that the perpetrator, rattled by I-na's encounter with Yoon-su, made a critical slip-up and accelerated their sinister timeline.
Impulsively, I-na embarks on a wild drive, and Han-saem, concerned for her safety, decides to accompany her. Her manic driving reveals a tailgater, but to his astonishment, it transpires that she hired the man to decode how the enigmatic killer tracks her movements. With the investigation hitting a dead end, Han-saem and Choi San revisit the crime scene. Suddenly, San notices an inexplicable absence: a handkerchief. He insists it was previously noticed by Yoon-su's feet but is nowhere to be found in the evidence or photographs. San, who arrived first, was followed by Su-gwang, Young-ha, Yang, I-na, and the forensic team. Han-saem issues a stern warning to keep this discovery under wraps and confers with Yang. Trusting San's integrity, Yang concludes that the killer or an accomplice must be embedded within the police force.
The sixth episode of Nine Puzzles culminates with I-na leading Han-saem to Mi-young's abode. She engages in lighthearted chatter about his mother and solitary life before dropping a bombshell. This apartment, once Yoon-su's, was bestowed upon Chi-mok, who subsequently gifted it to Mi-young. I-na senses that within these walls, secrets lie buried, waiting to be uncovered.
