In the early 1800s, the Robinson family, Father, Mother and sons Fritz, Ernst and Francis, flee Napoleon’s influence in Switzerland to settle a new colony in New Guinea. Their ship, however, is beset by pirates, and while the family is trapped below decks, the rest of the crew abandons ship near the dangerous reefs. The ship soon founders, but the family manages to break out and, upon spotting land nearby, fashion a raft to reach the shore. After leading the family in prayer, Father and Mother begin arranging for a crude shelter, not realizing that a tiger is watching them closely. The next day, the males return to the shipwreck to empty it of supplies. When they soon spot the pirate ship that had earlier attacked them, Father hoists a quarantine flag that dupes the pirates into a retreat. They load months’ worth of supplies, including goats, a donkey, birds and a pig, behind the raft, but the swimming animals attract the attention of sharks, who circle ominously. The boys are able to fend off the sharks by shooting at and stabbing them, and afterward, Ernst, the erudite brother, lassos a giant turtle that helps pull them to shore. On land, Mother searches for young Frances and finds him trapping a baby elephant. When the tiger attacks, the captain’s dogs, Duke and Turk, whom Frances insisted on rescuing from the ship, come to his rescue. Soon, the elephant is put to use hauling logs for the family as they plan a treehouse fort. Although Mother worries that they are becoming too permanently settled on the island instead of searching for escape methods, and further declares that she and Frances will stay away until the structure is perfectly safe, the men labor to make the house a home. As soon as it is finished, they bring Mother to view the luxurious quarters, complete with running water, retractable stairs and a master bedroom with a sky view, and Mother is overcome with admiration. In the days that follow, she adds feminine finishing touches to the house while Ernst reads the ship’s books and deduces that the island’s many different kinds of animals, such as ostriches, zebra, hyenas and monkeys, must be the result of an earlier land bridge connecting it with Africa. Although they want to sail around the shore to ensure they really are on an island, rather than a peninsula, Mother remains fearful of the risk. One day, however, while they play in the river among the waterfalls, Father maneuvers Mother into insisting that they must escape their idyllic new home for the good of the boys, and agreeing to let them explore for that purpose. Fritz and Ernst construct a small sailboat and set forth. The trip begins peacefully, allowing them time to discuss their future plans and desire to return to Europe to flirt with girls. Soon, however, they catch sight of the pirate ship and, in their attempt to hide, find their boat overturned and dashed against the rocks. They swim to shore, where they see two English prisoners, Capt. Moreland and his grandson Bertie, tied up while the pirates argue in an Asian language. The pirates approach the prisoners, but after Moreland offers to write a ransom note, they agree to leave Bertie alone. The pirates move away to form a plan, affording Fritz and Ernst a chance to sneak over and free the Englishmen’s bonds, but when the pirates spot them, Moreland insists they take Bertie and run. After a narrow escape, the boys manage to elude the bloodthirsty pirates, stealing one pistol along the way. They camp for the night, where Bertie declines to sleep with the brothers. The next day, upon reaching a river they must remove their clothes to cross, Bertie grabs the pistol and runs away, and when Fritz wrestles him to the ground, he discovers that Bertie is really Roberta, a young lady disguised as a boy to keep the pirates at bay. Soon, the brothers are competing to treat her as gently as possible, and Ernst though tries to impress her with literary quotations, Roberta is more attracted to Fritz’s quiet strength. Their trip across the island is long and arduous, and while traversing a swamp, Fritz is attacked by an anaconda. In the ensuing struggle, Ernst helps drive the snake away, but loses the compass. Fritz insists that he can lead them back, maintaining his confidence even when Ernst and Roberta threaten to turn around without him. Soon after, they come upon a zebra stuck in a mud bog. After chasing away the hyenas and vultures that circle nearby, the trio manages to free the zebra and leads it toward home. Meanwhile, Father, Mother and Frances try to put their worries about the boys aside to celebrate Christmas. When Father sings, Mother breaks down, but is thrilled to hear the sound of her oldest sons joining in the song as they find their way back to the treehouse. She welcomes Roberta and clothes her in a lovely dress, the sight of which inspires the brothers to further acts of competition. Days later, Father decides to demolish the shipwreck so it will not signal their position to the pirates, and the boys offer suggestions for defense, including moving to an uphill location, building snares and homemade bombs, stockpiling ammunition and, at Frances’ insistence, digging a pit in which to trap a tiger. During the preparations, Roberta proposes to Fritz that her grandfather could secure him a job in London, where she loves the society balls, but he replies that he would prefer to live off the land and help settle New Guinea. Although disappointed, Roberta, secretly a sharp-shooter, urges Fritz to teach her to use the ship’s rifles. Her plan almost works and they are about to embrace when Ernst sees them and jealously starts a fight with his brother, which Father interrupts. That night, Roberta tearfully apologizes to Fritz for causing trouble between the brothers, and vows to leave him alone, as they want different kinds of lives. Just then, to everyone’s shock, Frances’ tiger trap proves successful, and the family places palm fronds over the mouth of the pit to hide the tiger below. The next day, Father leads a race, with each person riding a different wild animal. When Mother shoots the pistol as a starting signal, however, the noise reaches the pirate ship at sea, and they soon come ashore to attack. The family rushes to their uphill position, and use all of their homespun weapons to battle the pirates. Their ingenious inventions, including the tiger pit, are successful, but there are so many pirates the family cannot sustain their attack, and soon are in desperate straits. At that moment, the pirates begin to retreat, and although the Robinsons are at first puzzled, Roberta spots her grandfather’s ship at sea and realizes he has returned to rescue them. At dinner in the treehouse, Moreland offers an ecstatic Ernst a position at a British university, and asks the parents whatever post they would prefer. Father is shocked and delighted when Mother declares that they would like to stay on the island with Frances, and Moreland asserts that, as the British will soon settle the island, Father is sure to be named governor. In private, Roberta tells Fritz that she understands Mother’s decision, and he agrees that he, too, would like to stay, as long as she will stay with him. This time, when Ernst catches them kissing, he quietly retreats, and later waves goodbye to his family, certain to return soon to visit their idyllic island.