Nobody Knows

Detective Young Jin, from the regional investigation unit, has only one thing on her mind: to catch the 'Stigmata' serial killer that murdered her friend 19 years ago. She carries guilt and trauma over the event. She has no friends and lives alone. Her downstair neighbor’s son, Eun Ho, has built a relationship with her ever since she moved above their apartment. He does menial house chores for her. One day he opens the door of the room he wasn't supposed to.
Sun Woo, the former chairman's son and the current chairman's brother-in-law, is Eun Ho's homeroom teacher. He feels something is wrong with Eun Ho.