King Rudolf IV (Sellers) dies in a balloon accident upon the celebration of his seventieth birthday. In order to secure the throne, General Sapt and his nephew Fritz travel to London, where the King's son, Rudolf V (Sellers), resides and lives through the day in London's pleasure establishments; but the King's demented half-brother Michael (Kemp), thinking that he is the better claimant, sends an assassin after them. Hansom cab driver Sydney (or Sidney) Frewin (Sellers), the new King's half-brother from an affair with a British actress, rescues Rudolf from an assassination attempt. Once his resemblance to the King is noticed, Frewin is hired by the general ostensibly as the King's coachman, but actually to play the role of decoy. The ruse is quickly uncovered, however, when during an attack by Michael's men the royal guardsmen address Frewin as their new king, and the two look-alikes get acquainted. In an unattended moment, Rudolf is captured and brought to Michael's castle of Zenda. Out of necessity, Frewin has to keep masquerading as the King for the coronation ceremony. Princess Flavia, Rudolf's fiancée (Frederick), is perceptive enough to see through the ruse, and after Frewin and the general have confided in her, she quickly becomes Frewin's trusted ally and love interest. Complicating the scheme on Frewin's side is the jealous Count Montparnasse whose wife (Sommer) has become infatuated with Rudolf, and on Michael's side by his mistress, Antoinette, who is wildly jealous about the prospect of Michael marrying Flavia and in turn is the love interest of the slightly unbalanced Rupert von Henzau, Michael's second-in-command. After several assassination attempts, Michael attempts to lure Frewin into a trap. While the trap fails, Frewin, acting as Henzau's coach driver, is recognized and captured upon arrival in Zenda. Frewin and Rudolf escape with Antoinette's help, and when Sapt and his men arrive at the castle, Henzau switches sides and aids Frewin and Rudolf