Frank and Joe Hardy receive an unusual assignment from their detective father. They are to "break into" the house of a Bayport neighbor, Malcolm Wright and retrieve a top-secret invention that the scientist had hidden in his study before leaving for California. The invention is in danger of being stolen and the boys race against time to beat the thieves at their own game. The young detectives soon discover that they are involved in a mystery far greater in scope than just retrieving the invention. Their investigations put them on the trail of a dangerous gang of jewel thieves and smugglers. When Joe is kidnapped, this incident starts Frank off on a chase that almost ends disastrously for him and his pals. The exciting capture of several members of the gang during a stormy night on Barmet bay and a perilous encounter with the insidious Anchor Pete concludes one of the most complicated cases of the Hardy's career.