The Jolley-Rogers and the cave of doom

Every full moon, a mysterious ship is seen off the coast of Dull-on-Sea. Then, the day after, gold and jewellery are missing from the homes, shops and museums of the town. Matilda needs the Jolley-Rogers' help to investigate. Can Matilda and Jim Lad retrieve the stolen booty from the mysterious ship before the moon fades? Or will they end up stuck on board with ghostly pirates until the next full moon?