Murphy is out posing as a criminal armourer to make an arrest. During the set-up he recognises Caz Miller, a man known for his connections to criminal gang leader and known cop-killer Dave Callard. Miller tells Murphy that he wants to kill another gang member called Richard Holloway. Murphy stops the planned arrest because he thinks he can gain Callard's trust and infiltrate the entire criminal organisation to take everyone out.

Callard recommends Murphy to the McGeechan brothers. They plan to reactivate some assault rifles, but won't tell Murphy why they need them. Murphy knows they plan something big, but first he has to make sure his cover isn't blown.

Murphy tries to get counterfeit Euros for Callard, and asks fellow undercover officer Jill Calder for help. She has infiltrated an organisation that is printing the fake money. Together they try to set up a deal for both their clients and plan to make the big arrest during the handover.

The operation against Callard builds to its conclusion as Murphy is hired to help complete a shipment of Afghan heroin into England. His undercover work could help secure the conviction, but Murphy's presence is demanded elsewhere, as a key prosecution witness in the High Court.

Callard starts systematically destroying the forensic evidence. Murphy and Miller fear that he may want to destroy one or both of them too.