"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...

A mysterious old pirate lures a group of rowdy sailors to an island with buried treasure, but there is a surprise awaiting them when they get there.