A hardened criminal is shot dead by an armed-response unit led by Sgt Danny Waldron, who claims they were acting in self-defence. But when AC-12 gather evidence suggesting it was a deliberate killing, DC Kate Fleming is sent undercover into Danny's team to find out more. She is quick to identify tensions within the team - only for her own conduct to come under scrutiny.
The shooting at the drugs house intensifies the scrutiny on Danny Waldron's armed-response unit, who once again close ranks. But with Kate Fleming on the inside, AC-12 feed in new information that helps her expose cracks in the team's story, while Steve Arnott looks deeper into the suspect sergeant's background and makes a shocking discovery. However, the anti-corruption cop's life is complicated by an investigation into his own past conduct.
DS Steve Arnott believes there is a dark secret in Danny Waldron's past. But while DI Cottan is commended for cracking the Waldron case, the spotlight falls on Arnott's questionable conduct in a previous enquiry.