The Secret of the Minstrel's Guitar

The Dana girls take time off from Starhurst School to accompany their Portuguese-American friend Isabel Sarmento and her father to Lisbon, Portugal. Senhor Sarmento has enlisted the assistance of the teen-age sister detectives in solving the baffling thefts of cork products from his Lisbon warehouse. But even before the ocean liner the Balaska leaves New York Harbor, Jean and Louise find themselves involved in a mystery on shipboard. What is the secret of the minstrel's magic music? Why is a thief so eager to get hold of the handsome young man's guitar? Is the dangerous schemer somehow connected with the warehouse thefts? Jean and Louise successfully combine sightseeing in beautiful, romantic Portugal with outwitting a tribe of sinister gypsies as they unravel this seemingly unsolvable double mystery.