The Five meet each other for the first time. Julian, Dick and Anne meet their cousin, George, and her dog, Timmy, at Kirrin Cottage. After an awkward exchange, they go and explore Kirrin Island where an old ship wreck is washed up after a thunderstorm. The wreck offers clues about treasure in the hidden tunnels in Kirrin Castle, but George's father wants to sell the island. But how will the children convince him not to do so?