Sung-Soo (Son Hyun-Joo) runs a successful cafe. He lives in an exclusive apartment complex with his wife (Jeon Mi-Sun) and two children. Sung-Soo holds a dark secret about his older brother and he also has a strong fear of germs. One day, Sung-Soo learns that his older brother has gone missing. He hasn't seen or spoken to his brother in years. Sung-Soo, having a strong sense of guilt, decides to check on his brother's apartment, which is located in a seedy, rundown neighborhood. He then notices codes scrawled along the apartments there, identifying the occupants and gender. Sung-Soo suspects his brother has marked the apartments there. Sung-Soo also meets Joo-Hee (Moon Jung-Hee) and her daughter, who live across the hall from his brother. When Joo-Hee learns who Sung-Soo is, she becomes terrified and pleads to him to make his brother stop peeping at her young daughter. Joo-Hee senses that his brother is always watching them. Later, Sung-Soo notices the same type of codes are now scrawled outside of the apartments in his apartment complex.