Originally stationed in Portsmouth to keep an eye on German zeppelin flights, Luke discovers a British naval officer who appears to be on very friendly terms with the Germans on the Kobold. Further investigation reveals that the officer is directing zeppelins that fly reconnaissance missions over the port to determine the whereabouts of the flagships of the English fleet.

Luke learns too late that the suspicious English officer is actually spearheading a German spy ring controlled by an elusive mastermind named Krieger. Luke's worst fears are confirmed when two ships are blown up at their moorings with all hands on board, as the guns of August announce the opening of the Great War.

The leader of MO5, Vernon Keller, directs Luke to follow the traitorous English officer. Together with his hard-hitting, hard-drinking sidekick Joe Narrabone, Luke sets out on the trail. He is led to a vicious band of Irish terrorists bent on throwing off British rule. The Irish have infiltrated the British naval stockyards and army ammunitions storehouses. A massive international conspiracy is revealed, poised to leave England defenseless against a German invasion.

A wave of denunciations descends upon England, as Luke's superiors try to ferret out the loyal subjects from the traitors. Knowing that the British authorities are on to him, Krieger throws his plan for the destruction of the English military into high gear. Now Luke must race against time to locate and neutralize each terrorist cell it can destroy its British target. In so doing he enters into a deadly cat-and-mouse game with the crafty Krieger, with the fate of Europe hanging in the balance.