Lee Na-Mi (Chun Woo-Hee) works as a marketer at a start-up company. She is an ordinary office worker who works hard everyday. She also works part time at her father’s cafe, Cafe Mizi.
One day, Lee Na-Mi loses her smartphone on the bus and it is picked up by someone. When Na-mi uses a friend’s phone to call, Oh Jun-Yeong (Im Si-Wan) uses a fake voice app to speak so his voice won’t be recognized, instructing her to pick up her phone at a repair shop. He does this so he can get access to her password and under the pretence of fixing her broken phone screen, he installs spyware on her phone.
Na-Mi unexpectedly walks around and does everything with her phone: every action, conversation and text being monitored by Jun-Yeong. Jun-Yeong slowly infiltrates into Na-Mi’s personal life, causing her father to be suspicious of him. Na-Mi expresses curiosity about Jun-Yeong as they seem to have a lot in common. He claims to work at a software company and seemingly tries to help her with the spyware.
Na-Mi is fired from her job after Jun-Yeong leaks confidential information and frames her. He also tries to make Na-Mi believe that it was her best friend who is behind the spyware. Ultimately Jun-Yeong succeeds in isolating Na-Mi from everyone important in her life.
Meanwhile, Detective Woo Ji-Man (Kim Hee-Won) investigates a murder case involving a dead body that was found on a remote mountain. There, he finds clues that points to his son Woo Jun-Yeong as the possible perpetrator. Seven additional bodies are found which points to Jun-Yeong who keeps the smartphones of his victims, the latest one, Eun Mi-gyeong, who authority believed had run away.
In a shocking twist at the end, Na-Mi and her father fight for their lives while Detective Woo Ji-Man uncovers the truth about Jun-Yeong.