While cruising around Barmet Bay in their speedboat, the Sleuth, Frank and Joe Hardy spot an unexplained submarine and are rammed by another vessel which speeds off. The boys are soon on the trail of the culprit, Pancho Cardillo, but their investigation is stalled when they are attacked by thugs hiding in Cardillo’s stolen boat.
Returning home, the boys hear of their father’s latest case. Fenton Hardy is investigating the disappearance of one Elmer Tremmer, a book-keeper due to testify against fraudsters selling worthless stock in Mexican firm Costa Químico Compañia. Before long the boys have two new clues to follow up. Firstly, a ring found in Cardillo’s recovered car bearing a strange insignia – a flaming bundle of sticks with a large superimposed letter “P”. Secondly, a typewritten note arrives through the post reading simply:
BEWARE OF THE MARK ON THE DOOR!
Before long the cases are intertwined and the Hardys, their father and their friend Chet Morton are off to Mexico investigating a criminal known as Pavura who rules through threat and fear and leads a strange cult of Mexican Indians.
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