Police anti-corruption unit AC12 return for another investigation. Sergeant Danny Waldron and his armed response team shoot dead a hardened criminal, but cracks soon appear in their story. Are they hiding a cold-blooded murder?

After the shooting at the drugs house, DC Kate Fleming uses her undercover role to increase the pressure on Danny Waldron's armed response unit. Meanwhile, a previous AC12 investigation comes under legal scrutiny.

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.

Fresh testimony launches AC-12 back on the trail of the Caddy, a corrupt officer with links to organised crime. But new disturbing evidence suggests the Caddy may reside within AC-12.

AC-12's loyalties are divided when DS Steve Arnott comes under scrutiny from his colleagues. With his career in jeopardy, Arnott turns to an unlikely source for help.

DS Steve Arnott is arrested on suspicion of murder but continues to protest his innocence. Does DC Kate Fleming's undercover operation hold the key to Steve's guilt and the identity of the Caddy?