The story’s protagonist, William Waterman Sherman, who has taught arithmetic to schoolchildren for 40 years, finally retires at age 66 and sets off on a balloon adventure. Gruff, smart, and scientifically minded, the bearded Sherman enjoys tremendously his flight across the Pacific, and he is delighted with the secret, wealthy village he finds on Krakatoa.

Though generally courageous, Professor Sherman is at first frightened by the constant volcanic rumblings underfoot, and during his balloon flight over the mountain’s glowing, smoky caldera, he feels terror and nausea. Sherman otherwise reacts with oohs and aahs at the marvels he witnesses on the island. His honest reactions add texture to his descriptions; these normal, average responses make his experiences easier for readers to understand and relate to.

Sherman gets his last name from the author’s mother, whose birth name is Florence Sherman. The professor wishes simply to be left alone to enjoy the peace and silence of the skies, but he discovers that his wish to travel is really a thirst for excitement. He bonds with the Krakatoans and their romantic sense of adventure; this helps him realize that, despite the dangers, his visit to the island simply increases his own yearning to explore the world.