Plot
A ship carrying a Swiss family from Bern—Father, Mother, and their three sons, who are relocating to a colony in New Guinea to escape the Napoleonic Wars—is attacked by pirates. Abandoned by the crew, the ship eventually grounds on rocks off an uninhabited island. The family makes their way ashore along with the captain's two Great Danes. Father, eldest son Fritz, and middle son Ernst salvage supplies and livestock from the shipwreck. Pirates locate the ship, but Father scares them off by putting up a quarantine flag, signaling Bubonic plague aboard.
The family soon discovers that the island contains a diversity of wildlife, including a dangerous tiger. To provide safety and comfort, Father, Fritz, and Ernst construct an elaborate tree house complete with a water wheel. Youngest son Francis collects various animals including a young Asian elephant, a monkey, and an ostrich. Ernst theorizes that the island may once have been part of a land bridge between Africa and Asia. As the family settles in, Father opines that, by going back to nature, they have found everything they need in life. Mother, however, worries that her sons will never marry or have families if they are not rescued, and consents to allow Fritz and Ernst to circumnavigate the island in a homemade outrigger boat and search for other settlements.
During their expedition, the brothers come across the pirates, who have captured another ship and taken its captain, Moreland, and its cabin boy captive. They rescue the cabin boy, but the pirates spot them before they can free Moreland, who insists they leave him since the pirates intend to ransom him. The brothers and the boy flee the pirates through the jungle, the brothers later learning that the "boy" is really a girl named Roberta. Moreland (her grandfather) cut her hair and dressed her as a boy to disguise her gender from the pirates. They survive an attack by a green anaconda, but become lost and fight over what to do. Fritz's strong personality wins in the end, and they decide to press on. They rescue a zebra from hyenas and a quicksand trap; using it as a mount, they arrive back at the tree house just in time for Christmas.
Anticipating that the pirates will come looking for Roberta, the family scuttles their wrecked ship to hide their location. They fortify a rocky clifftop, building defenses and booby traps. Fritz and Ernst become rivals for Roberta's affections. Believing that her grandfather will return for her once ransomed, she intends to return to London; Ernst is interested in going to school there, while Fritz would rather go on to New Guinea to build a home of his own. Despite this, a romance develops between Fritz and Roberta, and the brothers come to blows over her. To relieve tension, Father declares a holiday to be held. That night, Francis manages to catch the tiger in one of the pits that they have dug.
The holiday begins with a race, the boys and Roberta riding on various animals. The pirates, sailing nearby, hear the sound of the starting pistol and come ashore. The family retreats to their fort, and the attackers fall victim to their traps and defenses. Kuala, the pirate captain, demands that they hand over Roberta, while his men sneak up the cliff side and attack from the rear. As the family is about to be overwhelmed, a ship captained by Moreland appears, destroying the pirates and their ship with cannon fire.
Moreland offers to help Ernst get into a London university, and to take the rest of the family back to Europe or on to New Guinea. Father and Mother, however, decide that they would rather stay on the island and keep Francis with them for a few more years. Moreland speculates that the island will become a new colony, and that Father will be nominated to be its governor. Fritz and Roberta also decide to stay on the island, and the family waves goodbye to Ernst as he, Moreland, and the ship's crew set out for England.
