An Unexpected Killer takes a deep dive into homicide investigations through captivating interviews with potential suspects, in-depth examinations of the crime scene and re-enactments. Viewers follow experienced homicide investigators who believe they are inches away from solving the case, until they come across something unanticipated. Whether it's a new witness, an overlooked piece of evidence or even an innovation in forensic science, in each case detectives find a startling discovery that sends the investigation in a completely new direction in their search for justice.