An ambush of a police convoy results in three police officers dead and a protected witness seriously injured. The sole surviving police officer is Detective Inspector Lindsay Denton. Deputy Chief Constable Mike Dryden takes personal charge, assigning AC-12 to the case. Initially AC-12's most valuable witness, the suspicion soon arises that Denton could be their prime suspect. In the pursuit of corruption, how far will they go to uncover the truth?