Four children spend their summer holiday near a lake in the English countryside. With their mother’s permission, they sail their small boat, Swallow, to an island where they decide to camp and live like explorers.
On the island they cook their own food, build shelters, and have many small adventures. Soon they meet two other children, Nancy and Peggy Blackett, who sail a boat called Amazon.
The groups become friendly rivals and create imaginative games and battles on the lake, turning their holiday into a series of exciting adventures.