Tragedy strikes when a police convoy is ambushed and three officers are killed, with a witness in protective custody getting seriously injured.
The force's Deputy Chief Constable Mike Dryden takes charge and assigns anti-corruption unit AC-12 to the case. But Kate Fleming (Hawes) has to exclude herself as she trained one of the victims.
The sole surviving police officer, initially a key witness, soon becomes the prime suspect. Did she really lead the convoy straight into a fatal trap?
Line Of Duty, BBC2, 9pm