When Judy's cousin Roxy asks her to inspect some ocean-front property, part of which has been left to Roxy in her stepmother's will, Judy is only too glad to do it. It will be fun, she thinks, to go Sunday with Irene, Flo, and Pauline on the boat over to Fire Island, off the Long Island shore, and see how a resort looks in November. Even when they reach the apparently deserted, windswept island with its boarded-up cottages and incessant pounding surf, the girls are still in high spirits. The make their way to "The Sand Castle," the cottage Irene and her husband had occupied the previous summer; and Judy is delighted with the tower room, whose windows look out upon the vast expanse of ocean. But soon it seems as if some subtle evil force is at work, hidden in the gathering fog, threatening the apprehensive girls. They are horrified to learn that there will be no boat to take them back to the mainland until the following Sunday! Thoroughly uneasy now, Judy nevertheless refuses to believe that the mysterious woman in black, whom she glimpses momentarily, is the ghost of the woman who used to come back to the island to dig for her jewels, lost in the worst of the big hurricanes. How Judy, by keeping her head and using courage and common sense, protects her friends from harm and solves the mystery, makes a lively and spine-chilling story.