William Beddoes arrives in Lisbon to investigate financial interests for his American bank. He is mistaken by British embassy official Hatton-Jones for an agent on a secret mission concerning a fortune in missing industrial diamonds. Despite his efforts to prove his total ignorance regarding the gems, Beddoes is pursued by adventuress Aurora-Celeste, the paramour of the murdered man he supposedly is replacing, and by Steve-Antonio, a bogus Portuguese who describes himself to Beddoes as an amateur smuggler and who himself is pursued by Amy Franklin, a former acquaintance from the United States. Soon Beddoes and Aurora-Celeste are being followed by international spies, all on the trail of the hidden diamonds. After being arrested and bailed out by Hatton-Jones, Beddoes decides to ignore his amateur status and solve the mystery. Beddoes, Steve, Amy, and Aurora are invited aboard the yacht of Dr. Mathieson, an Englishman, who reveals himself to be the real thief and Hatton-Jones to be the real agent. Beddoes then engineers an escape and recovers the missing diamonds. He takes his reward money and reluctantly says goodby to Aurora-Celeste. She, however, is fully confident that they will meet once again; she has purloined his passport.