Season 1

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.

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.

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.

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.

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