Detective Young Jin, from the regional investigation unit, has only one thing on her mind: to catch the 'Stigmata' serial killer who murdered her friend 19 years ago. She carries guilt and trauma over the event. She has no friends and lives alone. Her downstairs 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, Eun Ho seems to have fallen off the roof of a building and left in a coma. Young Jin senses foul play and decides to take it upon herself to get to the bottom of it. With the help of Eun Ho’s homeroom teacher, Sun Woo, they both begin to uncover that Eun Ho’s injuries are no accident.