Sung Soo is a successful businessman who lives with his wife Min Ji and their 2 children in a luxury apartment. Life is mostly good, except for an older brother from whom he's estranged. One day, Sung Soo receives a phone call that his brother is missing and goes to his brother's apartment building for the first time in decades to look for him. There, he finds strange symbols carved into every door and meets Joo Hee and her young daughter. Joo Hee is convinced that someone is spying on them and lives in constant fear. Sung Soo figures out that the symbols carved onto the doors are "hide and seek codes" that indicate gender and number of people. Then when he returns home, he finds similar symbols carved into his door.