Like other police units, AC-12 catches criminals-but the offenders it investigates are fellow officers. When a police convoy is ambushed, leaving three cops dead and a protected witness seriously injured, AC-12 focuses its attention on the one officer who managed to escape the attack unscathed: Detective Inspector Lindsay Denton.

After a criminal is shot dead during a police raid, the AC-12 team begins examining the conduct of Sergeant Danny Waldron (Daniel Mays, The Great Fire), leader of an elite unit. But Waldron's loyal team obstructs the investigation, and the truth about the shooting proves more complicated than Arnott and Fleming could ever have imagined.

In a career-defining case, Roz is under intense pressure from her superiors to apprehend a serial murderer after months of fruitless investigation. When a young man is charged doubts around his guilt lead the chief forensic investigator to AC-12. Supt Hastings places DS Kate Fleming undercover inside Roz s team and with DS Steve Arnott piling on pressure from the outside, Roz is forced to act decisively.