When a series of terrible murders ensues in a residential area, Cha Su-yeol is forced to seek help from Jeong I-sin, a serial killer from the past.

I-shin offers assistance with the investigation, but it's unclear why she would help a cop. Su-yeol unravels the origins of I-shin's murderous instinct.

After a tense hostage exchange, I-shin confronts her protégé. Su-yeol makes a grim discovery about his family's fractured past.

The culprit begins to close in on Su-yeol's family. I-shin pressures Su-yeol to take action — but she happens to be the one threatening his family.

The culprit has been arrested, but the victim’s whereabouts remain unknown. Su-yeol struggles to save a life as I-sin reveals her true intentions.

Shaken by recent events, Su-yeol attempts to confide in Lee Jung-yeon. During a tense call, I-shin manipulates the killer into exposing a crucial lead.

Su-yeol finds I-shin in his home after she vanishes under police watch. The killer demands to speak to I-shin and gives her a hint about the plan.

As Jung-yeon copes with the truth of Su-yeol's past, the killer reveals their identity — and takes a hostage, forcing Su-yeol into a dangerous bargain.