The Prisoner of Zenda. I loved the movie so much that I was inspired to pick up the sequel, Rupert of Hentzau (picture from here).
The story picks up three years from the ending of Zenda. Rudolf Rassendyll has returned to England, leaving his cousin to rule as Rudolf V & to marry Princess Flavia. Flavia is unhappy but dutiful. She has never forgotten Rudolf & is still in love with him. Every year, Fritz von Tarlenheim, the narrator of the story & one of the inner circle who know about the events around the King's coronation, takes a red rose from Flavia to Rudolf. This is the only contact they have. This year, she decides to write a letter as well. The letter is stolen by Rupert of Hentzau, the dashing adventurer who was exiled at the end of the previous story. Rupert has never forgiven Rudolf or the King & has been waiting for a chance for revenge.
Rupert needs to get the letter to the King, hoping that he will divorce or exile Flavia. He enlists his hero-worshipping young cousin, the Count of Luzau-Rischeneim, to take the letter to the King. Fritz & Colonel Sapt realise the peril that the Queen is in & Rudolf arrives in Ruritania to defend the Queen's honour & face his old enemy once more.
The King has never recovered from imprisonment in the castle of Zenda & is peevish & weak. His character hasn't been improved by his adversity or the sterling example Rudolf presented on the throne. If anything, he has grown to resent any mention of Rudolf's name. Sapt & Fritz serve him loyally but they can't help but regret that Rudolf couldn't be King. Rudolf's arrival is the beginning of a convoluted plot where he once again impersonates the King so as to receive Rischenheim & get possession of the letter. Sapt has convinced the King to go to his hunting lodge. However, Rupert smells a rat & pursues the King to the lodge with devastating results. Sapt & Rudolf's servant, James, must come up with an audacious plan without any way of letting Rudolf & Flavia know what has happened & what they plan to do.
Rudolf, meanwhile, is in the capital, Strelsau. When he is recognized as the King, he has no choice but to continue his impersonation. He constructs an ingenious plot & discovers Rupert's hiding place in the city. The final encounter between these two is swashbuckling at its best. Rudolf's sense of honour & fair play would seem to be no match for Rupert's wily tricks & the sword fight is very tense. The denouement is sad & tragic but inevitable. There's really no
other way for the story to end & there's a certain rightness in the
way the story closes.