Although the narrative begins and ends in San Francisco and offers an intervening excursion to New York City, most of the action takes place on Krakatoa, a volcanic island west of Java in the Pacific Ocean. The year is 1883. The events of the plot are defined by the volcanic setting in many ways. An ominous rumbling permeates the land as the ground constantly shifts and settles. Because of the quivering landscape, people must acquire "mountain legs" to walk steadily, just as a sailor needs "sea legs." Krakatoa also boasts several rich diamond mines that keep the first settlers on the island in spite of the dangers.

In addition to a dramatic, towering volcano, du Bois's Krakatoa also offers many technological wonders, including the House of Marvels, a museum filled with wildly imaginative inventions such as a bed-making machine; and a balloon merry-go-round, consisting of seven boats joined together to form the rim of a wheel, each held aloft by a balloon.

The impending volcanic eruption generates tension and suspense. The reader wonders when the explosion will occur and how the islanders will cope. The events of the story move from the initial account of the Professor's rescue at sea to a lecture he gives before his beloved Western American Explorers' Club, in which he relates his adventures in flashback. By placing the Professor's fantastic adventures within the framework of the lecture, du Bois makes the events of the story seem more realistic.

The blend of fantasy and realism is an effective literary device. The details of life on Krakatoa are realistic, even though the concept of the isolated settlement is fanciful. The episodes involving the farfetched inventions recall Jonathan Swift's island of Laputa in Gulliver's Travels (1726). Like the narrator of Daniel Defoe's Robinson Crusoe (1719), the Professor accidentally ends up on a remote island, but in du Bois's tale the survivor is met by a man dressed in formal apparel, waiting to provide him with spats and a fresh collar.

Du Bois's style is lively and rich in concrete details. The author describes the fabulous diamond mines with a striking simile: "If the famous Jonkers' diamond had been tossed on the brilliant floor of the Krakatoa diamond mines, it would have been as impossible to find as a grain of salt in a bag of sugar." At times the sheer accumulation of details creates a humorous effect, as when the narrator lists the improbable array of items that he throws overboard when his balloon is damaged:

I threw chairs, table, books, water distilling apparatus, water cans, dishes, garbage containers, cups, saucers, charts, globes, coat hangers, clotheseverything noneatable. Clocks, scissors, towels, combs, brushes, soaps, everything I could lay my hands on I threw out through the doors, off the porch, out of the windows.

In The Twenty-One Balloons, du Bois pursues an egalitarian ideal. The depiction of the community on Krakatoa is a model of social equality. The families living there have learned to share their resources and responsibilities equally. They have established a government, arrived at by consensus and honored by all. Their considerable free time is devoted to mutually beneficial projects. When the Professor arrives in their midst, the families are constructing an amusement park, a shared aim with joint effort and commitment.

The cooperation, resourcefulness, and mutual respect that make the isolated community on Krakatoa a success, also make possible the safe escape of everyone from the island when the disaster finally strikes. The twenty families take enough food in their balloons for all to survive, carefully rationing it and fairly doling it out. They plan to jump...