While the Jolley-Rogers are enjoying a day at the beach, little sister Nugget toddles into a cave. When she doesn't come out again, Mum goes in to find her. And, when Mum doesn't return, Dad follows. Jim Lad is suspicious and sends Bones the dog to Dull-on-Sea to find Matilda before he too ventures in to the cave. Jim Lad soon discovers a magical haul of treasure inside the cave has bewitched his family. Will the cave's spooky inhabitant, a sinister sea witch, keep them prisoner forever, or can Matilda and Bones come to the Jolley-Rogers' rescue?
