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. Using Mr. Hardy's recently purchased scientific device, a sound spectrograph, the boys identify the voiceprints of the leader of the masked robbers. A chase ensues that takes Frank, Joe, and their pal Chet Morton to the grape-growing region of California and involves them in a dangerous game of hide-and-seek with the bank robbery gang, who also are searching for the broken saber. A startling discovery at a movie location leads to the solution of this thrilling, fast-paced mystery. - Flyleaf.