When Nancy searches through the knapsack of an amnesia victim, she finds an unusual ring. Before long, she is caught up in a second case about a beautiful harpist. As Nancy works both cases, her discoveries reveal an important connection between the hospital patient, the harpist, and troubling enemies from abroad.

When Nancy searches through the knapsack of an amnesia victim, she finds an unusual ring. Before long, she is caught up in a second assignment from a beautiful harpist. Nancy's discoveries reveal an important connection between the hospital patient, the harpist, and enemies from abroad.

When Nancy and her friends ride deep into the Sawniegunk Forest in search of a flying saucer, they find themselves in the middle of more than one mystery. Wildcats, runaway horses, deadly snakes, and a disappearing Indian keep the sleuths tangled in danger and suspense.

Nancy and her friends, Bess and George, meet Joanne Byrd on a train ride home. Joanne lives at Red Gate Farm with her grandmother, but if they do not raise enough money to pay the mortgage, they will soon lose the farm! Nancy, Bess, and George decide to stay at Red Gate for a week as paying customers. Soon, they learn about the strange group of people who rent a cave on the property. They describe themselves as a nature cult called the Black Snake Colony. Nancy investigates their group and helps to uncover a ring of counterfeiters in town!

In a response to a friend’s call for help, Nancy’s father, a lawyer, asks her to travel to mysterious Crocodile Island with her friends Bes and George to study the reptiles and try to uncover a group of suspected poachers. Upon their arrival in Florida, the girls are kidnapped but cleverly escape to pursue their detective work. Dangers mount as they cope with reptiles, enemy boats, and exciting chases after the men who are responsible for a sinister racket that involves many unsuspecting victims. In the end, Nancy makes a bold move to untangle the mass of clues, but she and Ned become imprisoned in the enemy’s submarine and are held for ransom!