Every full moon, a mysterious ship is seen off the coast of Dull-on-Sea. Then, the next day, gold and jewelry are missing from homes, shops, and museums all over 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 a crew of ghostly pirates forever?