Like all Enid Blyton books, this one is a delight.
From back cover: Adventure with a capital 'A' greets the Adventurous Four when they are shipwrecked on a group of lonely islands. The trouble starts when the Four discover strange and sinister men prowling about the islands at the dead of night. Living up to their name, the youngsters investigate -- but are spotted by the intruders.
A desperate battle is soon being fought over the beaches, cliffs, and caves -- and when the Adventurous Four finally discover what the men are upt to, it's enough to send the Air Force roaring into action!