In their motorboat, the Sleuth, Frank and Joe Hardy search Barmet Bay for a dangerous stranger who stole a valuable boat. Suddenly, in the eerie fog, they spot the craft drifting aimlessly out to sea. What happens next starts the young detectives and their pal Chet Morton on an intriguing adventure that takes them to Mexico and into the comparatively unexplored desert and mountain regions of Baja California.
What happened? They saw the periscope of a submarine!