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 DullonSea 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?