Synopsis: What if there was a stranger in my house...? Signals for hide and seek □1○1△2 Seong-soo (Son Hyeon-joo), 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 werid symbols all over the house and he meets Joo-hee (Moon Jeong-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. Someone else is living in my apartment with me! Seong-soo returns to his from his brother's and notices a familiar symbol written next to the bell of his house. A missing brother. Hide and seeks codes. A shocking truth for two different families and the struggle to save their families starts now.