An odd mark on a key case which the man sold to their mother leads the teen-age sleuths to an Indian village, whose chief begs them to help him. Two strangers have claimed title to the Indian's land, the deed to which had been secretly buried by the chief's father, along with other valuable tribal possessions, shortly before he died. The only clue to the location is that a crisscross shadow marks the site when the October full moon is low in the sky.

How Frank and Joe find the missing deed and the other Ramapan treasures, how they prevent the phony leather-goods salesman from carrying out a ruthless scheme, and how they help their father solve the top-secret case he is working on for the U.S. government makes exciting reading for all fans of the Hardy boys.