When a criminal is shot dead by a police armed-response unit led by Sergeant Danny Waldron, AC-12 gather evidence suggesting it was a deliberate killing rather than self-defense.

Danny's armed-response unit come under increasing scrutiny as AC-12 feed Kate information to help her crack their story. Steve makes a shocking discovery, then his own past catches up with him.

DS Steve Arnott believes there is a dark secret in Danny Waldron's past. Then the spotlight falls on his questionable conduct in a previous inquiry - the Lindsay Denton case.

Fresh testimony launches AC-12 back on the trail of the Caddy, a corrupt officer with links to organized crime.

AC-12's loyalties are divided when DS Arnott comes under scrutiny from his colleagues.

Steve is arrested on suspicion of murder but continues to protest his innocence. Kate's undercover operation unveils unexpected facts, but will it hold the key to uncovering the identity of the Caddy?