As Detective Woo Ji-man tracks the serial killings discovered in the mountains, he finds personal items and circumstantial clues indicating that the killer is his estranged son, Woo Jun-yeong. Ji-man attempts to resolve the mystery quietly to shield his family, only to find himself constantly outmaneuvered.
Meanwhile, the stalker uses his spyware access to systematically dismantle Na-mi's life. He poses as an IT specialist to gain her confidence, leaks sensitive company product details from her accounts to get her fired, and frames her best friend Eun-ju to leave Na-mi completely isolated. When Na-mi’s father suspects him, the killer breaks into the father's house, tortures him, and holds him captive.
The major narrative twist occurs in the third act: the killer is not Detective Ji-man’s biological son. He is a serial identity thief who murdered the real Woo Jun-yeong years prior and appropriated his name, phone, and life history. He maintains an encrypted digital catalogue of smartphones belonging to dozens of victims he has systematically stalked, exploited, and murdered.
In the climax at the father's residence, the killer traps Na-mi and attempts to drown both her and her father in a water-filled bathtub. Detective Ji-man and his partner arrive just in time. During the violent confrontation, Na-mi manages to seize the detective’s dropped firearm and shoots the villain herself, ending his reign of terror. Na-mi’s father survives, and the killer is neutralized.