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. But 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, Frank starts a search for his brother that nearly ends disastrously for him and his pals. It’s boat chases, rides in old and potentially unsafe biplanes, and much more.