Masterminds in forensic science, criminal psychology and law come together to crack the toughest police cases and bring dangerous criminals to justice. Jack Po "Pro Sir" (Wayne Lai) is a senior forensic chemist who now heads the police Forensic Division. Using his unparalleled expertise in criminal psychology and firearm mechanics, Pro Sir works closely with Mandy Chung (Maggie Cheung), the senior pathologist who is considered an expert in many fields, including psychology, anthropology, criminology and forensic science.