Professor Soames has invented an ultrasonic vibrational rifle that is capable of obliterating armoured vehicles. Intending to use the weapon to shoot down an experimental aircraft, criminal Mark Slater and his gang kidnap Soames' daughter, Anne. With the professor's assistance, Unwin and Harding rescue Anne and set out to thwart the plot.

While racing to intercept international assassin Sakov, who has arrived at the Greenways health clinic on the pretence of receiving therapeutic treatment, Unwin is unaware that the agent has designs on the new GK2 additive chemical that, mixed with water, produces a compound as combustible as high-octane fuel.

The sabotage of multiple military installations leads The Bishop to contact Unwin. Tracking down Brother Gregory, a vicar involved in a car accident near the site of the latest attack, Unwin helps Harding to slip into Brother Thomas' briefcase. Arriving at Pennydridge Seminary, Harding discovers that the vicars are in fact mercenaries who answer to their own Archdeacon.