Set in 1883, it concerns a schoolteacher/adventurer, William Waterman Sherman, who decides to get away from it all by building a hot-air balloon capable of supporting a small house and setting off on a leisurely round-the-world vacation. Less than a week into his journey, the balloon is forced to crash-land on the island of Krakatoa, where Sherman discovers that a group of eccentric billionaires have created their own tiny society based around gourmet cuisine and the shared wealth of a vast diamond mine hidden at the base of the volcano. Sherman becomes a permanent guest of the island, sharing both their secret and a portion of the diamond mines. Unfortunately, he has arrived barely a week before Krakatoa famously blows its top.
If the plot sounds familiar, it's because F. Scott Fitzgerald published "The Diamond as Big as the Ritz," which is essentially a much Darker and Edgier version of the same story, in 1922. Du Bois was informed of the similarity shortly before publication and swore up and down that it was sheer coincidence (even though he found it amusing that both he and Fitzgerald had the same dream of using their infinite wealth to be slid directly from bed into a hot bath). So far as anyone can tell, the resemblance really is just coincidence, and the lighthearted tone of The Twenty-One Balloons, as well as its target audience, is far removed from Fitzgerald's black-comedy adult parable about greed.
The Twenty-One Balloons contains examples of
- All Balloons Have Helium: Subverted. The mechanics and practicality of the various balloon inventions in the book are very accurately and carefully described. Some of them might even work in the real world!
- Alphabetical Theme Naming: All the Krakatoans have given up their real names and are known only by initials: father Mr. A, mother Mrs. A, children A-1 and A-2, all the way down to T.
- Balloonacy: Happens to a small child during Sherman's homecoming celebration.
- Cool Airship: Sherman's balloon-house is described in exquisite detail: perfectly engineered to be operated by one person for an indefinite amount of time, stocked with lightweight furniture and trimmed with balsa wood and bamboo. Even his mattress is made of balloon silk and filled with helium, so that it floats up to the ceiling for daytime storage.
- Equally cool but not quite as much fun is the Krakatoan escape platform.
- Cool Old Guy: Professor Sherman starts his adventuring life at age 66, having spent 40 years as a schoolteacher. He designs the aforementioned Cool Airship, survives two catastrophic crashes and one of the most powerful volcanic eruptions in history, and stubbornly refuses to tell his story to anyone but the members of his adventurer's club, even when the request comes from no less than the President of the United States.
- Eccentric Millionaire: The inhabitants of Krakatoa have chosen to exist apart from the rest of society, under a "restaurant government" complete with its own twenty-day calendar, with no responsibilities other than creating exotic and incredible luxuries for their own consumption.
- Gilded Cage:
- Sherman is told on arrival that he is now a permanent guests of Krakatoa. Subverted in that, once he sees the diamond mines, he is quickly overwhelmed with the same fever as the other Krakatoans and no longer wants to leave.
- Inverted: the lure of the diamonds also keeps the Krakatoans themselves from leaving, in spite of the fact that they are now trapped on an isolated, inhospitable island where all their wealth is worthless.
- Gentleman Adventurer: Sherman is a well-bred, well-read former schoolteacher who wants to see the world, but in relative style and comfort. Many of the Krakatoans have a streak of Gentleman Adventurer in them as well.
- Greed: Of a benign sort. The Krakatoans are essentially trapped by their own greed, unable to leave the island without the thought of its vast wealth haunting them, but they're more than ready to share their wealth with Sherman.
- Here We Go Again!: At the end of his story, Sherman plans to rebuild his balloon and continue his original adventure.
- Mistaken for Afterlife: After he awakens from his crash to find a white-clad man standing over him, Professor Sherman thinks he's in heaven. The man informs him that he's on Krakatoa.
- The Münchausen: Subverted. It seems as if Sherman will become a disbelieved ex-adventurer, but at the very end, just as his audience begins to express doubt, Professor Sherman produces the diamonds.
- Nuclear Family: All the Krakatoan families were selected to fit into this trope, in order to insure genetic diversity. All families have a mother, a father, and a boy and a girl within a particular age range.
- One-Letter Name: The Krakatoans all take alphabetical names from A to T. Sherman is discouraged from doing the same, as being called "U" would lead to pronoun confusion.
- Steampunk: Many of the island's inventions run on steam.
- Steampunk Gadgeteers: The entire M family. They're engineers who have outfitted their luxurious home with tremendously fun cutting-edge technology, all of which runs on steam.
- Threatening Shark: A school of sharks appears under the professor as his balloon is on the verge of crashing into the sea.
- Very Loosely Based on a True Story: Krakatoa is a real volcanic island that did explosively erupt in 1883, briefly encircling the entire planet with its debris.
- Worthless Yellow Rocks: The diamonds are completely worthless on Krakatoa, where everyone is equally wealthy and there is nothing to buy—and taking the diamonds off the island would render them just as worthless everywhere else.
