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.
A startling discovery at a movie location leads to the solution of this thrilling, fast-paced mystery. Recommended for ages 8 to 14.