In their motorboat the sleuth Frank and Joe Hardy search Barrmet Bay for a dangerous stranger who has stolen a valuable boat, Suddenly, in an eerie fog, they spot the craft drifting aimlessly out to sea. What happens next stars 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.

The search for the meaning behind a mysterious symbol that terrorizes the people of an entire village, a daring escape from a submarine, perilous encounters with a band of renegade Indians, an unusual smuggling operation-all combine to make this one of the Hardy's most exciting cases.