A wave of brutal murders grips a quiet neighborhood, compelling Cha Su-yeol to seek an unlikely ally: Jeong I-sin, a figure haunted by a history as a serial killer.

Though the perpetrator is in custody, the victim's location remains a mystery. Meanwhile, Su-yeol fights to preserve a life, as I-sin unveils the depth of her true intentions.

I-shin offers her aid in the investigation, though the reason for her assisting a police officer remains obscure. Su-yeol delves into the source of I-shin’s violent predisposition.

The perpetrator is drawing nearer to Su-yeol’s family. I-shin urges Su-yeol to act, unaware that she herself poses a threat to his loved ones.

After I-shin disappeared from police custody, Su-yeol located her at her home. The killer then made contact, demanding to speak with her and offering a cryptic suggestion regarding their scheme.

Disturbed by the recent turn of events, Su-yeol seeks solace in confiding in Lee Jung-yeon. Over the line, a palpable tension hangs as I-shin subtly guides the killer toward revealing a vital clue.

While Jung-yeon grapples with the reality of Su-yeol’s history, the killer unveils themselves, seizing a hostage and compelling Su-yeol to make a perilous deal.

The aftermath of the fraught hostage exchange left I-shin facing her student. In the quiet that followed the ordeal, Su-yeol made a stark and unsettling realization about the broken threads of his family history.