Unknown Person

Kyung-chul is a dangerous psychopath who kills for pleasure. Soo-hyeon, a top-secret agent, decides to track down the murderer himself. He promises himself that he will do everything in his power to take vengeance against the killer, even if it means that he must become a monster himself.

Two sisters return home after a stay in a mental institution, only to face disturbing events and a strained relationship with their stepmother. As eerie occurrences unfold, dark family secrets begin to surface, blurring the line between reality and nightmare.