Explosions at desalinisation plants in Africa and Burgossa point to sabotage. Unwin and Harding travel to the one surviving installation at Port Trennick and find themselves in a race against time to prevent its destruction due to a rigged underwater inlet.

George Gray, an inmate at North Exmanston prison, breaks out of his cell and contacts Sir Humphrey Burton, who has promised him a route out of Britain onboard a submarine. Unwin is tasked with intercepting the pair at Kew Gardens and apprehending them with the aid of the Minimiser.

The secret development of the new XK4 fighter aircraft has been jeopardised by De Groot, an expert in industrial espionage, who has blackmailed ornithologist John Masden into attaching miniature cameras to his domesticated pigeons for illicit surveillance. Investigating, Unwin and Harding discover that De Groot plans to use the birds to bomb Crayfield Airbase.