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DS Steve Arnott is transferred to the police anti-corruption unit after the death of a man in a mistaken shooting during a counter-terrorist operation. 

Season 3, Episode 1: 

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.