Police drama about the investigations of anticorruption unit AC-12. When DCI Roz Huntley captures a serial killer, the chief forensic investigator alerts AC-12 to a possible miscarriage of justice. The story follows the actions of AC-12, led by Superintendent Ted Hastings and DI Steve Arnott, as they investigate DCI Joanne Davidson and her team, including former AC-12 officer DI Kate Fleming.