Prologue - Prince Boris Sàndor, the penniless heir to the throne of Sylvaria, has fled incognito to Budapest to enjoy himself before marrying his cousin, the equally penniless Princess Rosemarie of Moreina. The clientele of the “Grille Américan” prefers the Charleston and other jazz music (which Sàndor hates) to the Csárdás and the waltz. Even Sàndor’s old friend, the Primás (gypsy violinists), has to play jazz.

Mary, the daughter of the millionaire, Mr. Lloyd, the Sausage King of Chicago, arrives with her father’s private secretary, Johnny Bondy, himself a millionaire. She and other members of the “Young Ladies’ Eccentric Club” are visiting Europe as part of a bet: whoever buys what is most difficult to obtain for money will win one million dollars. She wants to "dance the Charleston with the supposed heir to the throne” Sàndor, pretending to be the prince’s aide-de-camp, refuses on behalf of his master, but says he would be happy to waltz with her. An auction ensues in which Mary and Sàndor bid for the Charleston verses the waltz; she wins by having the Charleston played and assures Sàndor that she will dance the Charleston with the Prince before long.

Act I - Prince Sàndor celebrates his birthday by assuming the regency while his uncle, King Pancras XXVIII, is in Monte Carlo. He learns of Mary’s arrival in Sylvaria; she has come to teach the locals the Charleston, so he immediately outlaws it. Bondy informs the Ministers of Sylvaria that Mary plans to buy and modernize the royal Palace. On learning that she is prepared to spend six million dollars, they agree. Princess Rosemarie, who suffers from a lisp, enters and is told that she must marry Prince Sàndor. Neither of them is keen on the idea. Bondy and Rosemarie meet, she tells him of the dynastic marriage to poor Prince Sàndor, since no rich suitor would have her because of her lisp.

Sàndor learns of Mary’s plans regarding the palace and is furious. She counters by telling him that no doubt princes are just like a doll she once had - a fine uniform and nothing but straw inside. When Sàndor explains to Mary that the pile of junk that she intends to modernize is his childhood home, she realizes his true identity. He agrees to the sale but declares that every penny shall be used to improve the lot of his people. While he reviews a farewell parade, Mary cables her father that she has bought a palace and is determined to buy the prince who goes with it as well.

Act II - The palace has been modernized at great expense but, as Sàndor points out, tradition has been sacrificed to comfort. He still refuses to dance with Mary, whom he teases because she has been learning to waltz. She counters by pointing out that her jazz director has been received in audience everyday. Sàndor explains that having outlawed the Charleston, he feels obliged to find out about it.

The Ministers of Sylvaria would like to see Sàndor marry Mary. King Pancras agrees and makes her a duchess so that she can be Sàndor’s social equal. Bondy is rewarded for his part in the deal with the Star of David and the rank of Count. The only thing outstanding is the consent of Mary’s father. Mr. Lloyd arrived with members of the “Young Ladies Eccentric Club”, seeking an explanation of Mary’s cable. Unwilling to admit that she is in love with Sàndor, she tells him of the bet, but he understands the situation.

Bondy and Rosemarie meet once again. He asks her whether she would accept a rich count rather than a poor prince and she is delighted. Mr. Lloyd agrees to have a look at Sàndor, who is unaware that Mary is in love with him. At first Lloyd thinks the Prince nothing but a useless puppet, but later realizes that he is a man of character. The ladies from the “Eccentric Club” inspect Sàndor and decide that Mary has won the prize. Sàndor discovers that his ministers are planning to sell him to Lloyds of Chicago, is shown a copy of Mary’s cable and is enraged. She is proclaimed a duchess. Sàndor announces his engagement - to his cousin Rosemarie! Bondy is shattered, and so is Mary.

Epilogue- In the “Grille Américaine” King Pancras is annoyed that the rich heiress has not been captured for Sylvaria. She arrives with a mysterious stranger and on seeing her, King Pancras decided to marry her himself.

Sàndor, who is once again in Budapest, learns that his bride to be, Rosemarie, has eloped with a rich young American. He sees Mary with the stranger, who stares quite shamelessly at him. Summoned to explain himself, he tells Sàndor that he is the Chief Executive of Paramount Fox Films, where it has been discovered that life produces much better films than the scriptwriters. Mary’s elevation to peerage has provided the title for their latest film. The difficulty of Sàndor’s engagement to his cousin Rosemarie has been resolved when the Fox executive exposes that he is actually Johnny Bondy and the new husband of Rosemarie. There is nothing left to prevent the happy ending on which American moviegoers insist. Mary confesses her love, pointing out that now Sàndor will, after all, dance the Charleston with her. He replies that the Charleston is passé, but that he would be delighted to dance a slow foxtrot.