Nine Puzzles Episode 1 opens with a chilling scene of high schooler Yoon I-na returning to a dark, stormy house. With her parents gone, only her uncle Yoon Dong-hoon remains—until she discovers his dead body in a pool of blood, stabbed in the neck with an awl. A strange puzzle piece depicting a wolf in a rabbit suit lies near him. Rookie cop Kim Han-saem begins investigating and suspects I-na almost immediately. She lacks emotion and claims to have no memory between entering the house and finding the body. Her story includes a 13-minute gap unaccounted for, and the awl used in the murder matches one of three she purchased earlier—one of which is found in her art studio with her fingerprint.

Despite her calm demeanor, I-na’s inconsistencies raise suspicion. She and her uncle had reportedly argued, and her route home leaves a troubling window for a potential crime. Han-saem aggressively probes her alibi, and though she denies any motive, he remains skeptical. That night, I-na lies in the same spot her uncle was murdered and imagines committing the crime, even smiling through a vision of blood. The next day, she nonchalantly presents the puzzle piece to Han-saem and challenges him to solve the case.

A decade later, I-na is now a criminal profiler with a peculiar, detached personality. She thrives in adrenaline-charged activities like bungee jumping and appears carefree, even playful with her therapist. Yet, her dreams remain haunted by her uncle’s unsolved murder. She remains close to Han-saem, who is now a maverick investigator still obsessed with the case. Despite his unkempt appearance and brash methods, he’s respected for his commitment.

Han-saem investigates further, pressing an old witness from I-na’s school days who recalls I-na seeking an alibi around the time of the murder. Another awl was also reportedly seen in her dorm. Meanwhile, Dong-hoon’s old colleagues—including Dong-su, now a commander—still sympathize with I-na and view Han-saem’s suspicions as harsh, though they acknowledge his heart is in the right place.

As a profiler, I-na proves her skills by cracking a child murder case using psychological manipulation. While her coworkers accept her eccentric behavior, Han-saem continues retracing her steps from the night of the murder, perplexed by her choice to take a longer, CCTV-less route. Later, a new character—an unidentified girl in a red car—appears, and her movements are tracked around the city late at night, subtly linking her to something ominous.

The episode ends with another twist: a delivery arrives at Dong-hoon’s house just as the housekeeper leaves. I-na discovers a second puzzle piece, this one showing a serpentine woman speaking to a girl. Terrified, she tries calling Han-saem, but he doesn’t answer, leaving her and viewers on edge.