Nine Puzzles Episode 3 opens with I-na recalling her uncle’s murder, a memory that has long evaded her. Meanwhile, the media stirs controversy around the internal investigation. To hasten progress, Chief Hyun agrees to let I-na officially join the task force, despite Han-saem’s clear frustration. When she arrives at his precinct, the tension is immediate. She informs him of her memory, but Han-saem remains skeptical and accuses her of manipulation. During a therapy session, I-na confides in Dr. Lee that she’s been profiling Han-saem to decide if she can rely on him, while also doubting the accuracy of her recovered memory. Dr. Lee suggests that the trauma of Mi-young’s murder finally triggered I-na to remember.

As the detectives review Mi-young’s case, they find out that the regular who usually parked in her spot had moved due to repeated harassment. This allowed the murderer to park Mi-young’s car in the blind spot unnoticed. Later, when I-na explores Han-saem’s desk—mistaking Choi San’s for his—she finds delicate paper roses and quizzes Choi about Han-saem’s MBTI. She is somewhat relieved to discover that Han-saem had actually been thorough in Dong-hoon’s investigation.

Han-saem meticulously re-examines the CCTV footage and notices something alarming: the killer was already in the car, holding the rope, when Mi-young drove in. This leads him to Griffin, where there are no security cameras, confirming that’s where the killer entered the car. I-na, having reached the same conclusion, shows up too.

Taking things further, I-na invites herself to Han-saem’s home, where she charms his mother during dinner while subtly continuing her profile on him. She learns about his kind nature and his efforts to uplift a sick high school friend. As they leave, I-na pleads for Han-saem’s cooperation, suggesting he profile her as a way to build trust.

Meanwhile, Sergeant Yang uncovers a partial fingerprint on Mi-young’s headrest that was initially missed. They send it in for analysis. I-na, now officially part of the team, presents her serial killer theory tied to the puzzles. She argues that both Dong-hoon and Mi-young’s murders were cold, precise, and void of emotional flair—more like executions than crimes of passion. Still, the team isn’t convinced—except Han-saem, who acknowledges the logic behind her analysis.

Elsewhere, Sergeant Nam visits Dr. Lee, overwhelmed by guilt and haunted by Mi-young’s death. She sees the second therapist and is visibly unsettled.

Digging into Mi-young’s past, Han-saem and I-na learn she ran away years ago. Her only known relative is her ailing father, Mr. Lee, who has dementia. When they visit the nursing home, I-na pretends to be Mi-young, momentarily triggering recognition in her father. He remembers disliking her aggressive boyfriend, someone he says changed her for the worse.

Han-saem questions the cousin Du-cheol, who echoes the same story—Mi-young returned changed, and her boyfriend was a thug. At Mr. Lee’s house, I-na and Han-saem find an old wallet with a picture of the tattooed boyfriend, narrowing their lead.

Their conversation on the way back shifts to grief and unresolved emotions. I-na explains that she still mourns her uncle. Han-saem admits that he will acknowledge her loss if he can confirm she had no hand in the murder.

That night, I-na shares drinks with Dr. Lee, discussing how people inevitably change. This thought reignites her curiosity about Dong-hoon’s past.

Meanwhile, Han-saem visits Griffin’s manager and learns the boyfriend’s name—Kang Chi-mok. He had once been generous, but recently returned aggressive and demanding money, claiming credit for Mi-young’s success. She didn’t report him, fearing retaliation.

In a crucial development, Yang finds that the fingerprint from the headrest belongs to Chi-mok. The team moves quickly, raiding his restaurant and home, but Chi-mok is nowhere to be found.

Episode 3 of Nine Puzzles ends on an ominous note—a red suitcase submerged in a river, hinting that something, or someone, is about to be unearthed.