The Twenty-One Balloons stresses the themes of creativity, individuality, eccentricity, and cooperation. The fantasy of The Twenty-One Balloons is built around an actual historic event—the massive volcanic eruption that destroyed the Pacific island of Krakatoa in 1883. But there the connection with history ends. The Professor discovers that the inhabitants of the island have established a unique, Utopian society, which he seeks to understand. The story begins when Professor Sherman, who teaches high-school, becomes bored with his life and sets off on a journey in a hot air balloon called The Globe.
He hopes the wind will blow him and his balloon all around the world. To his surprise, he instead has a crash landing on the mysterious island of Krakatoa, an island full of diamond mines and enormous wealth.
The secret society of Krakatoa is based on values of greediness for wealth and inactivity. They believe their lives are perfect because they never have to worry about money. They live empty and unfulfilling lives and must learn the value of relationships, education and their own lives. They must learn how having extreme excess of money, or anything, is worthless.
Threatened with destruction, the Professor and the inhabitants must cooperate and discover a way to escape the island before the final explosion.