The Grand Duke of Abacco is heir to a small and heavily indebted Mediterranean island. The Grand Duke is trying to hide from the usurious money lender, Marcowitz, who demands debt repayment. One hope to improve the situation would be a marriage to the Russian Grand Duchess Olga, who sent him a letter saying she is determined to marry him despite not knowing him, and against the opposition of her brother, the Crown Prince of Russia.
Businessman Bekker offers a substantial sum of money to exploit a sulphur mine but the Grand Duke is worried it would have negative effects on his subjects. Bekker joins with local conspirators to organise a revolution against the Grand Duke. In addition, the letter from Grand Duchess Olga is fraudulently obtained by Marcowitz.
Thief-detective Phillip Collin, passing as Professor Pelotard, tries to retrieve from Markowitz incriminating letters written by Congressman Isaac. In the process, he also finds the letter from Grand Duchess Olga and replaces it with a fake. Collin asks Isaac for a 50,000 pound loan as his fee, and tells him that he will use it to speculate on Abacco's debt. The Grand Duke decides to go secretly to the continent to retrieve Olga's letter.
Meanwhile, Phillip Collin meets an unknown woman in a café, who asks him to help her hide from her pursuers. He willingly obliges, and soon finds out that she is Grand Duchess Olga and that her pursuer is her brother.
The newspapers report on the speculation on Abacco's bonds, on the outbreak of a revolution in Abacco and on the disappearance of the Grand Duke. All regular voyages to Abacco are interrupted but Olga, who now passes as Collins' wife, manages to charter a ship to take her to Abacco island. She agrees to take along the Grand Duke, whom she has not recognised, and who introduced himself as a supporter of the Grand Duke. Marcowitz boards the Russian Crown Prince's warship and convinces him to go to Abacco by showing him his sister's (fake) letter.
Once Abacco, and after a brief skirmish, the Grand Duke and Collin overcome the self-proclaimed president of the island nation and his accomplices. However further revolutionaries overpower the Grand Duke and start preparing for his hanging. Olga now understands who the Grand Duke truly is, and wants save him by paying off the revolutionaries. This plan is without success.
At that moment, the Russian Crown Prince takes control of the situation with his navy. Unfortunately, the Prince is inclined to have the Grand Duke hanged, for having sold his sister's letter. However Olga dismisses the letter as clumsy forgery. Collin gives back the authentic letter to the Grand Duke which allows him to refute the accusation. The Crown Prince orders an immediate marriage and Collin celebrates the success of his speculation on Abacco's debt.[2]
