The protagonist is the Hon. Rudolf Rassendyll, younger brother of the Earl of Burlesdon and (through an ancestor's sexual indiscretion) a distant cousin of Rudolf V, the heir apparent King of Ruritania (a Germanic kingdom situated 'twixt the German and Austrian Empires). King Rudolf is a hard-drinking, feckless playboy, unpopular with the common people, but supported by the aristocracy, the Church, the Army, and the rich classes in general. His political rival is his younger half-brother, Michael, Duke and Governor of Strelsau, the capital city. Michael has no legitimate claim to the throne, because he is the son of their father's second, morganatic marriage: there are hints, about his swarthy appearance and Rassendyll's taunting him as a 'mongrel', that he may be partly Jewish. Michael is regarded as champion of Strelsau's poor working classes, and also is popular with the peasants in the countryside. (The novel seems sympathetic, however, with those who would support the dissolute despot.)
When Michael has Rudolf abducted and imprisoned in the castle in the small town of Zenda, Rassendyll must impersonate the future King at the coronation. There are complications, plots, and counter-plots, among them the schemes of Michael's mistress Antoinette de Mauban, and those of his villainous henchman Rupert of Hentzau, and Rassendyll falling in love with Princess Flavia, the King's betrothed. In the end, the King is restored to his throne — but the lovers must part.