"The balloon seemed scarcely to move, though it was slowly sinking toward the ocean of white clouds which hung between it and the earth." Stranded on an uninhabited island in the Atlantic Ocean Harry Johnston & Charles Thorndyke will be rescued by a strange underwater craft and transported to a new world. Here, they encounter a race of humans physically and mentally superior to those in their world, living in a civilization scientifically advanced than their own. "...it was a wondrous scene. The building in which they were imprisoned stood on a gentle hill clad in luxuriant, smoothly-cut grass and ornamented with beautiful flowers and plants; and below lay a splendid city--a city built on undulating ground with innumerable grand structures of white marble, with turrets, domes and pinnacles of gold".