It is set in 1964, and features a British couple who find themselves held captive by an ancient race of pygmies dwelling beneath the Sahara desert.

The Sahara is being flooded to create a new sea when the protagonist of the novel, Mark Sunnet, crashes his private rocket plane into an island of what is currently little more than a large lake.

The story finishes with sunburn after years of subterranean life, and with establishing a new company based on the primitive but unique technology that the escapees brought with them from the caverns.

An early passage in the book describes the comic-military reactions of Germany, which in 1935 was under Nazi rule, towards potential violations of their airspace by the protagonist's descending rocket plane.

That suggests Wyndham considered it possible that the Third Reich would survive at least another 30 years into the future, an expectation of totalitarian regime longevity that is mirrored by his similar projections of the continued existence of the Soviet Union into the 21st century in The Outward Urge.