At a time when she has no job and no prospects, Christy Ogden receives an invitation to join an old friend on a sailing trip around the Channel Islands off the coast of Southern California. It is 1939 and the world is on the brink of war, but Christy needs a break. She knows little about sailing and does not know Jean Dryden very well, but she is determined to take the opportunity to get away from home. Her mother died a year before, but her father has not recovered from it. He has been laid off from work even though the Depression is supposed to be over. Christy feels guilty, but at least her father will not have to worry about feeding and clothing her for two weeks. It is a fateful decision. In Santa Barbara, Christy meets Jean and they take off in her beautiful sailing yawl, called the Queen, to sail to Santa Cruz Island almost immediately. For several days, Jean teaches Christy to sail, they anchor in several coves and get to know each other. Christy notices that Jean has many ways to avoid direct questions. Christy wonders why Jean asked her, a very slight acquaintance, to come along on this trip. Slowly, it comes out that Jean has a mission. She thinks that the Japanese are about to invade the mainland of the United States. As proof, she mentions a number of transmission she has heard on the marine band radio. When she hears another, she will discuss it with Christy. Meanwhile, they meet a number of men and women on other boats and are invited to dinner on a yacht, picnic on a beach and to the boat of a man that Christy instinctively dislikes. Al Melrose is traveling with Matt Price, whom Christy likes immediately and cannot understand why he is with Al, the crude boor. Discovering that Al is a Harvard graduate and an expert in International relations, makes him more curious, but no more likable. While Jean is still away from the boat at a picnic, Christy discovers a book on the shelf in the boat's salon. Called Riddle of the Sands and published in 1903, she begins to read this fascinating story. It does not take her long to realize that the plot is similar to the trek Jean and Christy have been living. In light of the evasions and half-truths Jean has told her, Christy does not mention the book yet. Jean finally reveals that the broadcasts she has heard are in Japanese and Christy is the only one she knows who is familiar with the language. When Jean asks why Christy learned the language, Christy tells her the story of her own great grandmother who came from Japan. When she finally hears a message, she is puzzled by the fact that it is spoken in stiff, poor Japanese. Their boat is searched one night when they are asleep and they leave the next morning to sail east to Catalina Island. Jean is sure that an old flame of hers, whom they met their first night out, is behind it all. Christy is not so sure. After a day at the west end of the island, they decided to go to Avalon, the only town on the island. It is on the east end and they sail along the south shore to get there. Soon they discover that their engine does not work and they are nearly smashed on the rocks at Little Harbor. Jean's superior knowledge of sailing techniques saves them. The next day they set out again to sail to Avalon. In the meantime, there has been another broadcast and Christy tries to translate it. Christy asks Jean if she has deliberately created this trip and this danger to follow the plot of the Riddle of the Sands, the book she found in the Queen,. Jean denies it vehemently. As they round the east end of Catalina, they are nearly knocked down by a Santa Ana wind blowing forty knots from the east. Again Jean's knowledge saves them and they are able to sail up to a dock without mishap, watched by a gaggle of tourists and locals on the dock. They hire a local mechanic to work on the engine and he reveals that it has been deliberately sabotaged. Just as they should be trying to find out who has done this, Jean