Seong-soo, a successful businessman who lives with the perfect family in a luxury apartment, has secrets and mysophobia about his one and only brother. One day, he goes to see his brother after a phone call about him missing and sees weird symbols all over the house and he meets Joo-hee who knows his brother. "Please tell him to stop looking at my daughter. Joo-hee lives alone with her daughter but lives in fear of someone watching them. Seong-soo looks around carefully at the old apartment and realizes the symbols mean gender and numbers of people.Seong-soo returns to his from his brother's and notices a familiar symbol written next to the bell of his house too...