During the school holidays, the Walker children — John Walker [Simon West], Susan Walker [Suzanna Hamilton], Titty Walker [Sophie Neville], and Roger Walker [Stephen Grendon] — are staying at a farm near a lake in the Lake District. They sail a borrowed dinghy named Swallow, and camp on an island they call Wild Cat Island, mapping out a summer of exploration, small adventures, and the easy camaraderie that comes with a close-knit group of siblings.
They soon meet two local girls, Nancy Blackett [Kit Seymour] and Peggy Blackett [Lesley Bennett], who sail a dinghy named Amazon and live in Beckfoot, a house up the nearby Amazon River. The Blackett sisters introduce themselves with a confident energy, proudly calling themselves the Amazons and flying the “Jolly Roger” from their yacht. The two camps quickly establish a competitive rivalry, with Wild Cat Island becoming a shared playground where territory claims and friendly skirmishes lay the groundwork for a deeper bond.
Despite the rivalry, the two groups decide to join forces against a common enemy — Uncle Jim [Ronald Fraser], known to the children as Captain Flint (a nod to Treasure Island). Uncle Jim is the Blacketts’ uncle, but he has withdrawn from the others’ company to work on his memoirs and has grown decidedly distant and unfriendly. His withdrawal creates a curious tension, as the children must navigate not only the sea and the island but also the shifting loyalties of the adults who should be guiding them.
To settle who should lead their budding alliance, the children stage a contest to see who can capture the others’ boat. The challenge pushes them to their limits: the Walkers undertake a dangerous nighttime crossing of the lake, a daring move that earns them a stern reminder from Mrs. Walker [Virginia McKenna] about safety and prudence. Yet the risk pays off, and the Walkers win the contest when Titty [Sophie Neville] seizes the Amazon at the critical moment, shifting the balance of power in their favor and turning the tide of rivalry into alliance.
After a playful, mock battle between Captain Flint and the children, Uncle Jim is brought to task for his elusive behavior and is made to walk the plank on his own houseboat, a symbolic act that seals his fall from grace in the children’s view. In the upbeat, post-battle feast, the groups come together to seal a pact of lasting friendship, recognizing that their shared adventures on and around Wild Cat Island have created bonds strong enough to endure beyond the summer. The story ends with a sense of hopeful unity, as the Walkers and the Amazons agree to be friends forever, their summer stay transforming into a shared memory that will outlast the holiday.
