Five Have A Mystery To Solve: The Famous Five (Book 20)

The Five are requested by an old lady to look after her temperamental grandson, Wilfred, as he was by himself in the family cottage. The Five move in and discover that there are many mysterious tales about an island close by called Whispering Island and that an immense amount of treasure was once hidden there but was discovered by no one. The Five set off to see if they can find the treasure but instead, find themselves in a bit of trouble. How will they manage to save themselves?