Nobody Knows (2020)

Detective Cha Young Jin, from the regional investigation unit, has only one thing on her mind to catch the 'Stigmata' serial killer that killed her friend 19 years ago. She has no friends and lives alone. Her downstairs neighbor's son, Go 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. Lee Sun Woo, former chairman's son and current chairman's brother in law, is Eun Ho's homeroom teacher. He feels something is wrong with Eun Ho.