During a mock trial at school, a student makes a shocking discovery. Yang Jong Hoon, a former prosecutor-turned-professor, examines the crime scene.

Two detectives teamed up to catch a serial killer who murdered their family. Moo Jin Hyuk's life spiraled out of control after his wife was murdered. He starts to put himself together after he meets Kang Kwon Joo, US-graduated voice-profiler, who lost her police father to the same serial killer. They work together on the 112 (emergency telephone number) call center team.