On her way home from get off work, Na-mi (Chun Yu-hee) loses her smartphone, which contains all her personal information. Joon-young (Im Si-wan) finds Na-mi's phone and returns it to her, but installs spyware on it. By tracking her daily life, he learns everything there is to know about Na-mi—her whereabouts, hobbies, tastes, work life, finances, and social networks—and gets close to her by hiding his true identity. Meanwhile, detective Ji-man (Kim Hee-won) discovers traces of his son Joon-young at a murder crime scene and goes undercover to investigate him, preparing for the worst. Na-mi is relieved to have found her phone, but it's not long before her daily routine is turned upside down and out of control. With her phone gone, her entire life is in jeopardy.