This is the sequel to 'The Prisoner of Zenda'. Five years have passed. The King has become jealous of Rudolf Rassendyll and suspicious of the queen (Flavia)'s feelings towards him. Flavia decides that this must be the last year in which she sends to Rudolf the single red rose that betokens her love, and therefore she also sends via Fritz von Tarlenheim, her letter of good-bye.
Count Rupert of Hentzau, banished from Ruritania after the incidents of the earlier book, is plotting his return. In furtherance of his scheme he obtains both letter and rose, and plots to place them before the King. Rudolf, Fritz and Sapt must prevent this at all costs… (Summary by Andy)