Nancy, Bess, and George are vacationing in beautiful Big Sur on the rugged California coastline. But soon after they check into the Opa Tourist Lodge, their cottage is ransacked, and Nancy makes a startling discovery: The popular tourist retreat is also a hotbed of deception and crime! Her investigation leads from the magnificent Caine Castle straight to the heart of a dark mystery. Deep underground lies a labyrinth of caves—the site, according to rumor, of a treasure in gold. But someone wants the truth behind the legend to stay buried forever, and the deeper Nancy digs, the more perilous her search becomes!