As soon as Nancy Drew boarded the ship sailing to New York, she was faced with a new and dangerous secret. A man on the pier was giving someone a message in deaf-mute language: BEWARE OF NANCY DREW AND NOT... Who is NOT? Could it be Nelda Detweiler, the young South African woman who shares a cabin with Nancy, Bess and Georgie? When Nancy learns that Nelda was accused of stealing a diamond bracelet in South Africa, she tries to find out if the girl was really a thief or if she was the victim of some kind of conspiracy. Then, when Nelda is mistakenly brought to Nancy's cabin, a chest lined with copper appears the first clue indicating that an international gang of diamond thieves is on board. Nancy soon realizes that she and her friends are in danger. Mysteries and intrigue haunt the girls on the ocean, but with her characteristic intelligence, Nancy solves the mysteries and defeats her enemies before the ship reaches New York.