Frank and Joe Hardy become involved in an intriguing mystery which revolves around their fencing master, Ettore Russo. Proof that Russo is the rightful heir to his grandfather's estate hinges on retrieving the guard end of a broken saber lost many years ago in California. The young investigators' quest is complicated by a bank robbery during which some of their father's important records are stolen. A chase ensues in the grape-growing region of California as the boys engage in a dangerous game of hide-and-seek with the bank robbery gang.