Nancy is faced with two confounding riddles at once once more. The first is a question about a priceless collection of rare books that Mrs. Horace Merriam hired an art dealer to sell. Has the dealer taken advantage of her instead? The puzzling theft of a lovely marble statue is the subject of the second mystery. The renowned young detective disguises herself and assumes a false identity in order to solve both mysteries. Despite these safeguards, danger follows Nancy around at all times. As Nancy gathers proof against a crafty gang of art thieves, she encounters a number of thrilling hurdles, including an attempted kidnapping, an almost fatal sailboat collision, and a meeting with a dishonest sculptor.