Kyung Min is an average female living by herself in a studio apartment. When she gets to her place after work, she finds her door lock cover left open. Slightly scared of this, she changes her password. However, just before she goes to sleep, she hears someone trying to unlock her door. Startled by this, Kyung Min calls the police. Yet, the cops only show annoyance and no sympathy towards the frightened Kyung Min. A few days later, she finds more hints of someone trying to enter her home, and a murder incident occurs. Realizing she’s not safe and that the police are not to be trusted, she starts investigating on her own. A door lock case left open, unknown fingerprints on the keypads, and a cigarette bud found in front of her door; someone’s here.