A Bona Fide Killer

Bo-na, Tae-sung, and their four-year-old daughter Yul are a typical happy and harmonious family. After a three-year parental leave, Bo-na returns to work and successfully completes her duties from day one, defying concerns. Meanwhile, Tae-sung, covering a drug case, learns a serial killer who disappeared four years ago has returned.

Tae-sung and the police are surprised by Kingfisher's return and resolve to finally apprehend Kingfisher for good this time. Tae-sung seeks information from Detective Dong-jin, who previously pursued Kingfisher. Meanwhile, a new employee appears at Bo-na's company, seemingly harboring an unknown secret.

Hyun-nam tells Bo-na why she came to Durumi Electronics to find her. A Sales Team workshop takes place, and Bo-na spends time away from Tae-sung and Yul for the first time since her marriage. While enjoying an unexpectedly pleasant time, an unforeseen incident occurs at the accommodation.

As Bo-na keeps working, Yul starts whining, and Bo-na struggles with balancing childcare and work. Meanwhile, people are enraged by child sex offender Pyo Gyu-cheol's release after serving his sentence. Tae-sung meets Dong-jin who is tracking Pyo Gyu-cheol. Dong-jin believes Pyo Gyu-cheol could be Kingfisher's new target.

Bo-na, knowing today is Tae-sung's family's memorial day, takes a half-day off and leaves early. She does her best, but her mother-in-law's berating continues. Misunderstandings build as Tae-sung helps her after work, leading to an unexpected turn. Meanwhile, Tae-sung begins investigating the suspicious death of a construction worker.

Bo-na and her team travel to a Gangwon Province casino to prepare for a Client Meeting. When their plans hit a snag, Bo-na must step in for the Meeting herself. Tae-sung, investigating a construction worker's death, heads to the casino where Bo-na is, while probing the construction company's Chairman. Meanwhile, suspicious individuals appear around Bo-na's Meeting target.

Eight years ago, in World Cup-feverish Seoul, Bo-na's team holds a Meeting hidden. In the same building, chasing a pickpocket, Tae-sung attacked, collapsing. Bo-na finds him, calls 119. Four months later, Tae-sung awakens, meets Bo-na, falls for her. Bo-na, exhausted by her 16-year mission, plans to quit.

After discharge, Tae-sung retreats to a secluded mountain lodge, where he coincidentally reunites with Bo-na. Heavy snow isolates them, and only the two stay at the lodge for a while. Sharing food and enduring the cold, they open their hearts and start to love. Meanwhile, released vicious criminals, fearing Kingfisher, try to return to prison.

Sergei, Bo-na's childhood friend and a mercenary from Russia, reappears, and the truth of The Gosiwon Case in Haegong-dong from four years ago begins to surface. Bo-na investigates Sergei's return to Korea. Meanwhile, Dong-jin learns that Jang Eun-hak, the culprit who murdered his parents, has been paroled.

Monitoring paroled Jang Eun-hak, Dong-jin thwarts his renewed attempt at murder. A chase ensues, and Jang Eun-hak is eventually stopped by Kingfisher. This event prompts Tae-sung to resume writing articles about Kingfisher. Bo-na, despite her worries about these changes, decides to continue her own work.

Dong-jin recalls that Tae-sung's assailant and Kingfisher were in the same building in the past. He immediately proceeds with an interview with the assailant, during which he obtains a clue that Kingfisher might be a woman. Meanwhile, Bo-na catches her mother-in-law's Voice Phishing culprit and receives recognition from her mother-in-law. Although the family becomes harmonious, Tae-sung's anxiety steadily intensifies.

Bo-na's painful orphanage childhood surfaces, hinting at a past forged by violence and silence. Tae-sung faces a growing dilemma as doubts about someone close to him become harder to ignore. The task force closes in with a bold new lead, but an unseen hand seems to always stay one step ahead. What Tae-sung uncovers may destroy everything he loves.

A new murder case occurs, and it confuses the police, but they are sure that it was done by Kingfisher. Bo-na realizes she's framed, and the real culprit is Leila, her old colleague who is resentful toward Bo-na for past condemnation. The appearance of Leila makes Bo-na fearful for her family’s safety.

Leila is identified as Kingfisher, but inconsistencies arise. Meanwhile, Bo-na realizes that Tae-sung knew about her identity as Kingfisher. He confesses his love, but she leaves. During her absence, Tae-sung tries to clear Kingfisher’s name, shifting public opinion. He finds Bo-na, convinces her to reconcile, promising to learn her completely.