In Nine Puzzles Episode 6, Han-saem is shocked to find I-na moving into his apartment, declaring it their new base camp. Flashbacks reveal them drinking and bonding over suspicions of a mole inside the police force. I-na cozies up to the idea of staying, even bringing coffee, which leaves Han-saem reluctantly charmed. She notices his tattoo, and he admits it was from an undercover operation. She calls it cool, boosting his ego. They shift focus to the next lead—Do Yoon-su, DH Construction’s president—who had received calls from Chi-mok the night of his murder. Despite claiming ignorance, Captain Yang recalls Yoon-su denying knowing Chi-mok, which sets off alarms.

I-na visits Yoon-su, who arrogantly maintains he doesn’t know Chi-mok, Mi-young, or Dong-hoon. To rattle him, she recounts a chilling story of a taxi driver who killed a passenger and suggests a clean kill via a needle to the neck. Her tactic works, unsettling him. Meanwhile, Han-saem revisits Yang-hee, who remembers visiting Yoon-su with Chi-mok years ago to ask him to officiate their wedding. Offended, Yoon-su refused. Chi-mok, however, had said he and Yoon-su shared good memories—likely hinting at their fraudulent past.

After therapy, I-na jokes with Dr. Lee about her roommate, Dr. Hwang, and notes his painting looks like a knock-off. She later reveals to Han-saem that she finally slept soundly—for the first time in ten years—at his place. He’s exasperated as she brings more of her things over, but they shift back to Yoon-su. Their theory? The “good memories” were actually shared cons. Then, Yoon-su turns up dead.

His building is swarmed by reporters due to corruption rumors. Yang grows suspicious of Han-saem, who recently inquired about Yoon-su, but Han-saem brushes him off. The murder scene reveals a visible needle mark—clearly intentional, as if the killer wanted the cops to connect the dots. Yang’s journalist friend suspects the case is tied to the mayor’s lobbying scandal and the Yangyang Resort.

As I-na and Han-saem await the autopsy, they receive a fourth puzzle piece—a clown. The autopsy confirms insulin overdose. While wondering how the killer found them at the hospital, I-na races home to examine the puzzle. Alongside the clown, there’s a doll getting shot, cotton candy, dancing kids, a sandcastle, and a ferris wheel. She starts suspecting an amusement park is key.

Han-saem scolds her for running off, but she teases him, believing the killer wants her to solve the puzzle, not harm her. Meanwhile, Captain Yang questions if Han-saem is dating I-na, especially as they now arrive together. Han-saem blushes but implies their bond stems from Dong-hoon’s murder investigation.

They investigate Yangyang Resort, still under construction, where site manager Seo Do-ho reveals that Yoon-su was tight with Kang Chi-mok. Turns out, Chi-mok would drunkenly nag Yoon-su about developing a new resort—revealing that their history ran deeper than previously admitted.

Elsewhere, I-na visits Dream Land—the amusement park from the puzzle—which is shutting down. The owner denies any tragic incidents. Back at home, Han-saem discovers photos of Dream Land and lots of balloons, triggering his gut instinct: the killer is rushing because of I-na’s rapid progress.

Suddenly, I-na goes on a high-speed drive. Han-saem tails her and confronts a suspicious man, only to discover she hired him to figure out how the killer tracks her. She’s trying to outsmart her stalker.

With the Yoon-su case stalling, Han-saem and Choi San recheck the crime scene. San realizes a handkerchief seen by Yoon-su’s body is missing. It’s not in the evidence or photos. The only people at the scene before forensics arrived were San, Su-gwang, Young-ha, Yang, and I-na. Han-saem keeps this quiet and tells Yang, who agrees San wouldn’t lie. They now suspect the killer—or an accomplice—is a cop.

The episode closes with I-na taking Han-saem to Mi-young’s apartment. She chats about his mother and living alone before dropping a bombshell: the apartment originally belonged to Yoon-su, who gave it to Chi-mok, who then gave it to Mi-young. I-na believes this very place holds the next piece of the puzzle.

In Nine Puzzles Episode 6, I-na and Han-saem piece together that Dream Land may be the common setting for the puzzle killer’s motive. But a more immediate concern is that the killer—or their inside informant—is likely within the police force, suggested by missing evidence at the crime scene.