The story element driving the mystery is the Smoke of Eternity, a chemical/electrical reaction that dissolves anything it comes into contact with, such Coffern’s body, a bridge that Doc crosses, along with the surrounding water, a vault, and so on.
The mastermind behind all this is a criminal known only as Kar. He has a large gang and infrastructure; so much so he kills a dozen of his own men to keep them from talking to Doc Savage.
The Land of Terror also involves an expedition to Thunder Island, somewhere off the coast of New Zealand where it is believed that the mysterious Kar discovered a new element that creates the Smoke of Eternity.
The chemist, Jerome Coffern, along with a guy named Gabe Yuder and a taxidermist named Oliver Wording Bittman, had traveled to Thunder Island; Yuder was prospecting for oil. The island is described as being not much more than a large volcano devoid of any organic life.
The story involves traps, gunfights, murder, robberies, and dinosaurs, with Doc Savage and his pals striving to thwart the evil mastermind and avoid being struck by the mysterious Smoke of Eternity.