The day Mr. Machado's strange letter arrives from Portugal marks the beginning of an exciting trip and a baffling mystery for the Bobbsey twins to solve. Mr. Machado has written to ask the whole Bobbsey family to come to Portugal and pick up a rare and valuable gift he has for them. But the second page of his letter is missing, and the Bobbsey's have no idea what the gift might be. The only clue lies in the words "galo verde." To make everything even more puzzling, when the Bobbsey's arrive in Lisbon, they find that Mr. Machado has left the country! With some difficulty, the twins locate the housekeeper, who says there was indeed a package left for them, but it has been stolen from the house during a recent wave of burglaries in the neighborhood. The Bobbsey's don't speak Portuguese, but they manage to make themselves understood. With the help of some mew little Portuguese friends, Bert and Nan, and even Flossie and Freddie, track down one clue after another and have many adventures - some amusing and some spooky. In the end, it is little Flossie Bobbsey who, quite by accident, stumbles upon the secret passageway which, in turn, leads the twins to the hidden treasure.