Moo Jin-hyuk (Jang Hyuk) is a popular detective who solves big cases, but after his wife is murdered by a serial killer, his life spirals downward. He is filled with guilt for not being able to protect his wife and secretly searches for the killer. Meanwhile, Kang Kwon-joo (Lee Ha-na) graduated from the police academy with top marks. She starts working at the 112 emergency call center. While working there, a brutal murder case occurs and her father, a police sergeant, heads to the scene. Her father is killed there and Kwon-joo hears what happened over the phone. Afterwards, she goes to the US to study and returns to Korea as a professional voice profiler. Detective Jin-hyuk and Kang Kwon-joo solve cases together in order to track down the serial killer responsible for the deaths of their family members.