The film is based on Ransome's book, but the script incorporates many changes to the plot. It chronicles the story of the Walker children on their holiday in the Lake District in the summer of 1935. They want to camp on an island in the lake. When they get there in the boat ''Swallow'', they soon discover they are not alone. Two other children, the Blacketts, also known as the "Amazons", have set up camp there, and a battle for the island begins. But with Britain on the brink of war and a "secret agent" looking for the Blackett children's uncle, they have a series of adventures quite different from their plans.