In 1839, Johann Strauß is a celebrated waltz composer and the charismatic leader of a Vienna-based ensemble whose performances light up the city's cafe culture. His elder rival, Josef Lanner, commands his own orchestra with a firm belief in a more traditional, grand style. The two maestros clash over musical ideas and credit, with Lanner openly suspecting that Strauß copies melodies from him. Lanner’s daughter, Katti Lanner, is secretly in love with the timpanist, Gustl, but her father vehemently disapproves, setting up a personal rift that mirrors the public feud between the two composers.
Two emissaries from the British royal court travel to Vienna to study the waltz and bring its new flair to London; among them is Ilonka, a former lover of Strauß. While Lanner works at the piano on a fresh waltz, he has [Johann Strauß] write it down on Strauß’s monogrammed handkerchief, a symbolic crossing of their rivaling legacies. The night of a rehearsal turns tense when Lanner insults Strauß, prompting Strauß to storm off as several of Lanner’s musicians also exit. The two orchestras still share the same space, performing on adjacent pavilions until a heated clash erupts into a brawl. The British delegate, nevertheless, is impressed by Strauß’s energy and flair, which helps him secure a chance to take the music to London.
In London, Queen Victoria seeks a match with Prince Albert, but his willingness to commit remains hesitant. She invites him to a ball that features daring, near- scandalous close dancing, hoping the spectacle will tilt his heart in her direction. Meanwhile, [Katti Lanner] leads a women’s orchestra to London in an effort to defend her father’s reputation, and [Ilonka] returns to Strauss with plans that further complicate the romantic and artistic web. A butler summons Strauss to instruct Victoria in the waltz, while Gustl steps in, posing as Strauß, to guide her dancing. The performance unfolds with gusto, and Katti’s group plays Strauß’s melodies from outside the ball, prompting a summons for a private audience with Victoria. Although Victoria is impressed by Katti, she is denied a shared stage, as Strauß refuses to perform alongside a women’s ensemble.
As the ball hums with anticipation, Gustl is asked to give an impromptu, spirited waltz in honor of the royal engagement, and, in a moment of heat and improvisation, he discovers Lanner’s new composition on Strauß’s handkerchief as he wipes his brow. The royal court warmly receives the piece, yet Katti feels slighted by the treatment of her father’s work. Near the end, Strauß returns and, hearing Gustl play the newly uncovered waltz, contemplates taking credit for it himself.
A courtroom confrontation follows as Lanner sues Strauß for copyright infringement. The judges are evenly split on whether the waltz’s style belongs to Lanner or Strauß. Susi, Katti’s harmonica-playing friend, and Katti herself scheme to coax Strauß into admitting the composition is Lanner’s in front of witnesses. But Gustl shifts the narrative by pointing out that he, not Strauß, actually carried out the unauthorized performance, which changes the potential punishment. In the courtroom, Strauß concedes that the piece is Lanner’s and accepts full blame to shield Gustl from consequences. The crowd’s loyalty splinters into factions for Lanner and Strauß, erupting into a hubbub when a jail term looms. To calm the tempers, Strauß and Lanner begin composing a new waltz together, signaling a thaw in their rivalry, while Katti and Gustl reconcile and look toward a renewed partnership.
