A gang that uses parties as a cover for robberies victimizes a masquerade party Nancy is attending, so the teenage detective switches identities with a friend to solve the case.
A brass-bound trunk mistakenly delivered to Nancy's cabin on a New York-bound ocean liner becomes the first clue leading her to suspect that international jewel thieves are aboard.
Braving a series of dangerous situations, Nancy attempts to solve the mysteries of a singer being cheated out of royalty payments and a buried treasure to which the only clue is half of a locket.
A young detective's attempts to uncover a group of poachers on Crocodile Island in Florida involve her with kidnapping, reptiles, enemy boats, and a sinister racket.
Nancy, Bess, and George investigate after they find a yacht with a safe full of jewelry and a man who has been missing for ten years reappears to claim the fortune.