A chilling discovery unfolds as a seemingly innocent keyring from Ha-bin reveals the possibility of a serial murder case. Tae-soo grows increasingly anxious, sensing that Young-min and Ha-bin are hiding something crucial. Just as the tension rises, the police arrive at Tae-soo's home once again, intensifying the fear of what secrets might be uncovered.

Tae-soo gets a call from his wife, who died one year ago. As Tae-soo learns about Ji-soo's shocking secret, Joon-tae becomes increasingly apprehensive as the investigation progresses. Meanwhile, Ha-bin, who believes her mother is a murderer, begins to dig deeper into her mother's past as an unidentified man follows her from behind.

A man who claims to be Song Min-a's dad appears before Ha-bin and demands to know about her relationship with Choi Young-min. Ha-bin learns from him that Tae-soo helped Young-min escape from the police. The police raid Young-min's place and take all the kids for questioning. Ha-bin finds where Young-min is and hears who killed Lee Soo-hyun and why.