In this engaging film adaptation that draws from a novel, we follow the adventures of young Roger Walker, who is seen playing with toys in a bathtub, albeit without the water, while wearing a patch over his left eye. As the story unfolds in 1935 Portsmouth, England, the Walker family prepares for a journey by train, amidst the backdrop of the father serving on a destroyer in the South China Sea, leaving the mother responsible for their five children. Roger's age is not explicitly mentioned, but he is clearly quite young compared to his older siblings, John and Susan. The enthusiastic Tatty, always brimming with imagination, frequently states, "I am nine and a half," and occasionally serves as the movie's narrator. Vicky, the youngest sibling, is just a baby.
Before boarding the train, Roger accidentally drops his stuffed animal, which is promptly retrieved by a menacing-looking man accompanied by another individual wearing sunglasses, who warns Roger to be more careful. As the journey continues, the children eagerly unwrap gifts from their father, where John receives a knife that Roger is deemed too young to handle, while Roger's gift is a spyglass. While their mother consumes a cigarette, a rude man jolts into her, later intruding into the children's compartment. This mysterious man picks up Vicky and warns the children not to disclose his presence to anyone, adding an element of threat that Tatty interprets as pirate-like behavior, dubbing him Captain Flint. The tension rises as the man leaps out the window and clings to the side of the train, while the sinister pursuers that Roger noticed earlier traverse the top of the train, one carrying a gun, compelling the fleeing man to leap off.
Upon arrival at their destination, the family is met by the kindly Mr. Jackson, who takes them to their accommodation, a place they hope is familiar from their previous visits. However, Mrs. Jackson fails to match her husband's warmth, continuously chastising him. Eager for adventure, the kids—excluding Vicky—forego changing into casual attire, and instead race to the hilltop to gaze upon an island in the expansive lake, dreaming of a camping expedition. At the same lake, they spot an abandoned-looking houseboat, which piques their curiosity until John's stones break a porthole, provoking the anger of the man observed earlier on the train. Nearby, the men who were tracking him speak in Russian, heightening the intrigue surrounding their visit.
In a twist of fate, Jim Turner, a travel writer, arrives to recount the earlier events to the Walker family. Although the children's mother dismisses any wrongdoing, Jim produces the rock marked with a pirate insignia, which forces John to confess, albeit as a mere accident, and he apologizes while his mother promises restitution for the damage. Mr. Jackson later introduces the enthusiastic children to his sailboat, The Swallow, guiding them through the essentials of sailing and setting up camp. Though Roger, feeling left out due to his age, is initially unable to join in this thrilling experience, his swimming lessons soon grant him a spot on the crew—under the stipulation of receiving permission from their father.
Excitedly, they head to the post office, learning that a telegram is the fastest way to reach their father; however, lurking nearby are the suspicious individuals pursuing Jim, seeking a room under dubious circumstances. With parental consent finally obtained, the children embark on their long-awaited adventure. John assumes the role of captain, with Susan as second-in-command, Tatty tasks herself with the ship's log, while young Roger takes on lookout duties. But disaster strikes when their food basket capsizes into the lake, an unfortunate event leading to their declaration of "Walker Island" as their camping locale—their plans for a campfire dashed since the matches were lost with the food. A distant fire flicker ignites hope, only furthered when a cake crafted by Mrs. Jackson fortuitously washes ashore.
As night descends, tension rises when John and Tatty vanish, leaving Susan and Roger to discover a stern warning sign proclaiming that Amazons do not welcome trespassers on the island. They encounter two gruff men, both named Billy, who are aware of Jim’s connections and relay a message, also offering the kids invaluable survival tips on lighting a fire—along with matches just in case. Arriving back at their camp, John, who had taken the law into his own hands, confronts Jim about some suspicious documents spotted on his boat, fueling speculation about Jim’s true intentions.
Their escapades lead them to an encounter with Nancy and Peggy Blackett, occupants of a nearby sailboat named The Amazon, who claim jurisdiction over the island. A friendly rivalry emerges as the children ‘war’ over the ownership of the boats, setting the stage for playful confrontation. Amidst swirling events, tension escalates when Jim discovers someone has tampered with his boat. The discovery of John's knife aboard compels the children’s mother to intervene, bringing her to the island, where the gravity of their mischief becomes clear.
Eventually, the sinister men pursuing Jim transfer into action as Jim is confronted by their leader, who involves a gun while asserting that Jim's benign travel writer story is a facade. The situation spirals further when John attempts to rescue Jim, bearing his newfound bravery but also facing his doubts about shooting. Jim sagely advises John, saying it requires more courage to refrain from violence than to give in to fear. Just as the situation becomes dire, the kids use ropes to tether the men’s airplane, which, although not halting takeoff, buys enough time for Jim to escape and overpower his foes. With the men apprehended, a triumphant Jim embraces the Walkers, and they revel in their newfound friendship, culminating in spirited games of pirates, including the humorous act of walking the plank.
