Hwang Si-mok (Cho Seung-woo) is an exemplary Prosecutor who suffered from hypersensitivity to certain frequencies of sound. After undergoing a corrective surgery, he lost his sense of empathy and lacks social skills. While investigating a murder case, he meets Police Lieutenant Han Yeo-jin (Bae Doona), who assists him in solving it. As they begin to unravel the mystery behind the murder, they also discover that their efforts are being continuously foiled as expounding them will also lead to a large corruption scheme between the Prosecutors' Office and a private conglomerate.
In the second season, an ethical dispute arises within the Prosecutors' Office and the National Police Agency, with the former wanting discretionary power over all investigative proceedings while the latter seeks for an autonomous authority. In the midst of their respective agencies' conflict, Hwang Si-mok and Han Yeo-jin team up to independently conduct their own investigation on a concealed case