The Twenty-One Balloons (1947), written and illustrated by William Pène du Bois, is a fantasy-adventure novel for middle-grade readers. It tells of an adventurer who travels by balloon to the island of Krakatoa in 1883—the year the island explodes—and finds a secret colony of people made wealthy by diamond mines on the island. Filled with whimsical moments and ingenious inventions, the book won the Newbery Medal.

Author Pène du Bois was born into a family of artists and designers, was educated in France, and became an award-winning illustrator and novelist with over two dozen books to his credit. He illustrated works by Jules Verne, John Steinbeck, Roald Dahl, and others. He also was a founder and editor of the literary magazine The Paris Review.

The novel’s Lexile reading level is 1070L, suitable for advanced 5th-grade readers and above. The ebook version of the 2005 Penguin/Puffin edition forms the basis for this study guide.

Plot Summary

On the Atlantic Ocean, a ship sights an elderly man on the water, clinging to a wooden platform and 20 huge balloons. The crew rescues him, and he tells them he’s Professor William Waterman Sherman. He refuses to say how he came to be adrift until he can return to San Francisco and tell the story to his Explorers’ Club. News of the rescue travels fast, and the world wants to know Sherman’s story. The US president provides his personal railroad car to transport the professor from New York to San Francisco, where the mystery man arrives to great acclaim and a parade.

At the Club, Sherman describes his adventures: Yearning for a year-long, solitary trip aboard a giant balloon, he retires from teaching math, orders a balloon maker to create one to his specifications—including a small basket house attached to the balloon—and sets off on his expedition.

For a week, he enjoys sailing through the sky, peaceful and alone. Too soon, he sights an island. To climb past this bit of land, he throws garbage overboard, and seagulls grab it. One bird takes leftovers to the top of the balloon, but other gulls fight over it and tear a hole in the balloon. The craft sinks toward the ocean. As sharks swim below, Sherman throws out everything he can to lighten the load and reach the island. He grabs onto the balloon and cuts away his little house, which crashes into the sea. He just barely makes it to shore. Exhausted, Sherman falls asleep on the beach.

He’s awakened by a man dressed elegantly in a formal European suit. The man gives Sherman a similar suit to wear and then guides him inland through the jungle to a beautifully manicured lawn. The man’s name is Mr. F.; he says they’re on the volcanic island of Krakatoa. The ground shakes often from the rumbling mountain. Mr. F. shows Sherman a cave filled with diamonds. Sherman is amazed, but he’s told he can never leave because the island’s residents don’t want the world to know about their riches.

He sleeps at Mr. F.’s large French mansion. For breakfast, they visit a British-style home, owned by Mr. and Mrs. B. Everyone who lives on the island is there, dining on British cuisine. Mr. F. explains that Mr. M started the colony after being shipwrecked there and discovering the diamond mines. He returned to San Francisco and found 20 families willing to join him on Krakatoa. Once there, they agreed to keep the mines a secret, but quickly they quarreled over its ownership. Reason prevailed, and shares became widely distributed.

Each family now is named for a letter, A through T: Mr. and Mrs. A and their children, A-1 and A-2, Mr. and Mrs. B and their children, and so on. Assisted by the others, every family has built a large house, designed in the style of a country that matches the family name—C for Chinese, D for Dutch, E for Egyptian, F for French. All houses contain restaurants with food from the country represented by the family: A for American, B for British, I for Italian, M for Moroccan, and so on. Each day, all the inhabitants eat together at a different restaurant, moving in order from A to B to C until they reach T, when they start over. Their calendar also contains 20 days, A through T.

Everything on the island comes from supplies purchased on trips to other lands and paid for by diamonds. Mr. F. shows Sherman many of the ingenious devices built into the houses. Sherman wonders how the Krakatoans would escape if the mountain became too violent. Mr. F. shows him a wide platform with 20 folded balloons attached to hoses from hydrogen tanks. The craft can be inflated in 10 minutes and carry all 80 residents away to safety.

The professor gives an afternoon lecture on the latest doings in San Francisco, everyone’s hometown. Before he can finish, the mountain begins to shake violently, and everyone hurries to the escape platform. They lift off and pull away shortly before Krakatoa explodes. The platform wobbles from the shock waves and then drifts across the ocean to India, where all the families except one parachute to safety. The F family stays with Sherman, keeping the platform stable as it sails across Europe. The Fs jump off at Belgium, and Sherman, who has no parachute, ditches the platform in the sea, where he is rescued.

The Club audience gives Sherman a 10-minute ovation. One attendee asks what Sherman will do next. He shows them cufflinks made of large diamonds that he received at the island: With these, he’ll order a new balloon and continue his year-long voyage through the air.