Four children go to stay in a cottage near the sea where they meet Wilfred, an orphan who can attract animals by playing his pipe. They explore a nearby island for treasure and find the owner is being robbed by his gamekeepers. Brought before Sir Hugo the children convince him of their story but the gamekeepers lock them all up. They find a secret passage leading to the legendary crusader treasure and also find their way back to the castle. Again they are caught and left in a dungeon flooded at high tide. Wilfred plays his pipe and a badger appears showing them the way out. The tables are finally turned: the gamekeepers are handed over to the police and Sir Hugo decides to adopt Wilfred.

Four cousins meet a new friend who has strange powers over all the animals on Whispering Island. After discovering the crusader's treasure, they are imprisoned in a flooded dungeon.