When Robert and Clara Jordon are robbed of more than a million dollars worth of jewels, Nancy goes undercover as a member of a traveling carnival to prove the innocence of a young man with a police record. The Jordon's are certain that Henry Salazar is the thief, but Nancy and her father, lawyer Carson Drew, are sure that the now reformed criminal isn't guilty. Nancy joins the carnival to gather clues to find the real robber, and finds more than she bargained for in the after hours of the shadowy midway.