My biggest nitpick with the story is that without his trademark character, Mr. Rassendyll, Hope seems to be at a loss as to how to move the story along. As Rassendyll does nothing significant until the latter half of the book, you can imagine how that makes reading the first half. It feels like the beginning of this book was written by a different person than wrote the first book and the latter half of the second book. Overall, I was very disappointed when the author couldn't even get me to care about what happened to the various characters.
Prisoner of Zenda closed with Rassendyll safely in England once again, and the real King Rudolf on the throne once again wed to Queen Flavia. But Flavia loves Rassendyll, and risks everything to send him a letter. This is captured by Rupert of Hentzau (the dashing villain who escaped unharmed in the first book), who holds it above the heads of the conspirators to force them to do his bidding. This brings Rassendyll from England, and the game is on! Through it all, Sapt, Fritz, the Queen, and Rudolf must balance on a tighrope, for the king has been killed by Rupert. It is up to them to somehow bring things back into alignment without Rupert exposing the letter and destroying the Queen's honor.
I must say, as disappointed as I was with the writing, the ending was at least surprising. Still, the author seems to want to do everything he can to keep the Queen and Rassendyll apart forever, and the ending finally accomplishes this goal. I was not disappointed (For you'll remember I wasn't even attached to the characters), but rather slightly pleased to see a fiction novel which dared have a sad ending, especially one which had such a cliche storybook happy ending looming in front of it. So if there is one redeeming feature in the book, it would be the end, which leaves both halves of the illustrious double King Rudolf dead.
If you've read the first book and enjoyed it, and you don't mind slogging through a mediocre book, then go ahead and read Rupert of Hentzau. Otherwise, don't waste your time. Or, better yet, go read Prisoner of Zenda and just pretend there is no sequel.