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.

As part of an operation to seize £1 million in bank notes en route to London, the train loaded with the shipment is diverted to the disused Buffler's Halt Station. Assigned to protect the consignment, Unwin and Harding help to subdue the criminals responsible. However, back up the line, stationmaster Albert Hobson realises that he cannot stop the train, which is moving at 80 mph.