At a time when the profiling world was still foreign, Jang Tae Soo voluntarily took the path of a criminal behavior analyst. His proficiency gleamed as he extracted confessions from notorious serial killers. His composure and instinct for "doubt" in the interrogation room became his greatest weapon as an investigator. However, at the same time, he was a horrible father. When unexpected tragedy struck, he couldn't fulfill his duty as a father or husband. His high school daughter, Jang Ha Bin, quickly realized her gift growing up, seeing how the questions her father would ask criminals came naturally to her. The biggest obstacle in her life was her father, Profiler Jang Tae Soo. Ha Bin's lies did not work on her father, and as her lies grew, distrust between them grew deeper. Now, the father and daughter plunge deeper into the darkness as their doubts about each other conflict endlessly.