The title refers to a legendary land in the clouds described in medieval French chronicles where "aerial people" were said to dwell.
The first half builds Vallée's theoretical case, moving between medieval theology, psychology, and folklore to demonstrate that modern "aliens" are the same entities once called gods, angels, or monsters.
The book's central premise is that modern UFO encounters are part of a persistent phenomenon that has been with humanity throughout history, appearing in different forms depending on the cultural and technological context of the time. Vallée draws striking parallels between contemporary reports and historical folklore: