Chief Inspector Strock gets the tough cases. When a volcano suddenly appears to threaten mountain towns of North Carolina amid the non-volcanic Blue Ridge Mountains, Strock is posted to determine the danger. When an automobile race in Wisconsin is interrupted by the unexpected appearance of a vehicle traveling at multiples of the top speed of the entrants, Strock is consulted. When an odd-shaped boat is sighted moving at impossible speeds off the New England coast, Stock and his boss begin to wonder if the incidents are related. And when Strock gets a hand-lettered note warning him to abandon his investigation, on pain of death, he is intrigued rather than deterred.

Set in a period when gasoline engines were in their infancy and automobiles were rare, and when even Chief Inspectors had to engage a carriage and horses to move about, the appearance of a vehicle that can move at astounding speeds on land, on water - and as later revealed, underwater and through the air - marks a technological advance far beyond the reach of nations. It is technology invented by and for the sole benefit of a man who styles himself (with some justification) "The Master of the World."

This book is a sequel to an earlier Verne novel, "Robur the Conqueror", but enough detail is given to fully appreciate this story without having first read the other. (Summary by Mark)