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 Bess 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. She and Ned become imprisoned in the enemy's submarine and are held for ransom!
How Nancy and Ned are saved and the tables turned on the owners of Crocodile Island are left for the reader to discover.