While the Jolley-Rogers are enjoying a day at the beach, Dad and Mom and little sister Nugget are mysteriously drawn into a cave. When they don't return, Jim Lad is suspicious and sends Bones the dog to Dull-on-Sea to find Matilda before he, too, ventures into the cave. Jim Lad soon discovers a magical haul of treasure inside the cave that has bewitched his family. Will the cave's sinister sea hags keep them prisoner forever, or can Matilda and Bones come to the Jolley-Rogers' rescue?