Mom, Dad, Billy, and Kathy Masters move to Everett, Washington, to live in an old mansion that was built by Clifton Masters, Kathy and Billy's Great-Grandfather. The whispered stories of big sailing ships, pirates, and hidden treasure, excite Billy like thing has ever done before. Mother sees the old run-down house that sits well off the road atop a high cliff overlooking Everett Bay as needing a lot of work. Father sees it as a hobby, but Billy and Kathy see it as their own private Disneyland castle. Rumor has it that their Great-Grandfather was a pirate, and that he'd hidden a pile of gold somewhere on his estate. A neighbor, Old-Man Parker, adds weight to the rumor when he shows the children a gold doubloon. Hidden rooms and mysterious clues lead Billy and Kathy on an adventure of a lifetime, as they secretly search their Great-Grandfather's mansion for the lost treasure.