Sam Ryan looks into the death of Maggie Collins who died of a heroin overdose. Sadly, her body wasn't discovered for several days and in that time, her infant daughter died of dehydration. Soon after Maggie's death Stephen Jackson, a small-time dealer and addict, is killed in an explosion in his home. The autopsy reveals that both of them died of overdoses from the same batch of heroin and Sam begins to doubt that their deaths were accidental. When another small-time dealer is killed, the police think they have either vigilantes or a turf war on their hands. What they have is something far more complex that involves one of their own.

Are the deaths linked to a drug war or to vigilantes?

The accidental fatal shooting of a Polish teenager during an armed police operation prompts a series of murders linking Turkish gangsters, the grieving family and a fractured firearms unit.

When the body of an armed police officer is found, the team join the search for the killer. Jack discovers a link to London's gun trade and puts himself in danger to discover the truth about the death of the young Pole.

On Christmas day the team is called to the home of haulage company owner Andy McMorris to find three corpses and Andy disappeared. Sniper shots ring out, wounding DI Gibbs and McMorris's blooded daughter Mel tells Nikki her father shot at her and her horse. A large number of weapons are found in the home suggesting illegal gun importation whilst the diary of Mel's sister Stevie expresses hatred for her father and his estranged wife Zoe. DCI Cooke, a friend of the family assumes the case but appears to be hiding something and Clarissa discovers that the 999 call reporting the shots was made over twenty miles away.

As the three corpses are identified Vernon Rye, a friend of Andy's father Martin, one of the victims, is pulled in after admitting to a fight with him but Mel, recovering in hospital, tells Zoe and Nikki she heard Andy and Martin rowing before shots rang out. Zoe's sister Jackie is another suspect having had an affair with Andy, but claims she left the house before the murders. The discovery of a missing lorry full of guns and another body proves Andy's criminal activities and reveals who made the phone call. Cooke thinks he has found the killer but the pathologists' evidence proves otherwise.