Plot summary (original edition)

The Hardy boys investigate the disappearances of locals from a Mexican village.

Plot summary (revised edition)

While boating on Barmet Bay, the Hardy boys spot a periscope from a submarine and then collide with a rented speedboat driven by a Mexican man named Pancho Cardillo. Mr. Cardillo later returns to the rental location and steals the speedboat, leading the boys on a search for the stolen boat. While investigating Mr. Cardillo, the boys find an Indian ring with a strange crest on it and Fenton Hardy receives a threatening letter telling him to "beware of the mark on the door!"

The Hardy boys, their father, and their friend Chet Morton fly to Mexico where they find a band of Indians and a strange oil smuggling operation using submarines. Their deductions lead them to a small Mexican town where they learn that local people are mysteriously disappearing and the strange crest appears on the doors of people who have disappeared. The Hardys find a man by the name of Tico who is a great navigator and helps the Hardy boys find the culprit's hideout where they learn the real man behind the scheme is a man named "Pavura", which means terror in Spanish.

In the end, the Hardy boys and their friend Chet manage to stop the smugglers from getting away and solve the mystery of the disappearing Indians, all while solving the oil smuggling case that their father was working on.

Original edition (1934)

• Ghostwriter: Leslie McFarlane (?)
• Summary: The boys go to Mexico with their father in search of a missing witness in a court case involving fradulent oil stocks.

Revised edition (1967)

• Revision type: Completely rewritten; many plot elements retained.
• Ghostwriter: Tom Mulvey