In Nine Puzzles Episode 5, the twisted brilliance of the puzzle killer begins to take shape just as another disturbing death shakes up the narrative. The episode kicks off with a chilling visual—Chi-mok’s dismemberment illustrated on the third puzzle piece, ominously delivered to I-na. She begins to piece things together, realizing that Chi-mok was actually the killer’s target, not Yang-hee, who had unknowingly executed the crime. When I-na visits Yang-hee in prison, the widow finally grasps how she was manipulated. She and Jae-wook carried the suitcase to the lake, unaware that the puzzle killer had planted fake wheel tread marks to lead police straight to it.

The episode pivots to a fresh case involving the suspicious death of Choi Young-han, a factory worker who had just returned from a year-long sabbatical. Choi was found dead after a drinking session with three coworkers—his soju spiked with nicotine. As I-na investigates the clean, almost surgically precise crime scene, she runs into Sergeant Nam and attempts to bond over their shared therapy group, but Nam brushes her off publicly. Meanwhile, Han-saem remains hostile toward I-na, leaving her puzzled by his refusal to profile her.

A Suicide in Disguise

The murder suspects include Jung-chul, who discovered the body; Gap-je, who left early after announcing his exit; and Deok-mun, who brought the soju despite not drinking and was seen visiting a mistress. All three blame each other, but none appear to have motive. When I-na and Han-saem inspect the break room together, Han-saem gets inspired by his detective novels, and I-na pulls him into a roleplay session at his apartment to act out the suspects’ behaviors. By the end, they suspect someone else entirely.

Their investigation leads to Janitor Yeom, who smokes and dislikes break-room parties. He seems harmless but reveals a key incident from the past: Choi’s son, Dong-kyung, died a year ago. Han-saem keeps this information from I-na initially, but after she finds out, they work together and visit Choi’s home—eerily clean and void of life, except for a single photo of the four coworkers. The scene reminds Han-saem of a suicide attempt by a sick friend, leading I-na to the realization: Choi didn’t just die—he killed himself, framing the others as revenge for his son’s death.

The Truth About Dong-kyung’s Death

Flashbacks confirm this theory. Dong-kyung was left alone to operate dangerous machinery while the three coworkers took a break. When his arm got caught, it was too late. They tried to cover it up before calling authorities. Present-day interrogations show the trio consumed by guilt. Another flashback shows Choi preparing the poisoned soju and dying in front of them, staging the moment to look like murder.

In therapy, I-na is agitated by what she calls Choi’s poor planning. Dr. Lee counters by reflecting on the psychology of people like Choi—and I-na—who internalize pain and opt for self-destruction over retaliation. I-na bristles at being seen as “soft,” but the remark plants a seed.

The Puzzle Killer’s Motive Comes Into Focus

Acting on her introspection, I-na breaks into Han-saem’s apartment again, this time with a bottle of soju. She asks him to guess what kind of person she is—killer or victim. Han-saem, finally seeing her vulnerability, drinks the soju and survives. It was a test. She smiles, knowing he finally trusts her.

More importantly, I-na proposes a groundbreaking theory. The puzzle killer’s victims—Dong-hoon, Mi-young, and Chi-mok—were all morally questionable. The killer may not be choosing innocent targets, but punishing people who hurt him. The finished puzzle at the episode’s end, revealing a mysterious face, signals that there may be six more “culprits” yet to die before the killer’s message is complete.