After Amadeus is dismissed from the Salzburg court for his wild and disrespectful behavior, his sister Maria Anna faces an even worse fate: an arranged marriage by their father, Leopold. To avert their mutual ruin, the siblings secretly flee to Vienna, hoping to secure Amadeus a new position at the court of Emperor Joseph II. What Maria Anna doesn't know is that her brother is suffering from a wrist injury, which he is treating with a mixture of laudanum and alcohol.
Maria Anna's appearance is a triumph! The Emperor commissions Mozart to compose a folk opera, much to the displeasure of Marie Antoinette, who fears the decline of the monarchy and her own influence because of her brother's affinity for the people. However, the siblings' euphoria is short-lived, as Constanze's mother learns of her daughter's affair with Amadeus and throws her out of her boarding house.
Constanze was accidentally shot during the duel. After tending to her, Amadeus is arrested. When the Emperor learns the reason for the duel, he announces a public aria competition to decide who will compose the national opera. At court, Marie Antoinette tries to seduce Amadeus, who flees at the last moment. The pressure becomes too much for him, and he sinks deeper and deeper into his addiction.
Maria Anna narrowly escapes being exposed – only to be cornered by the Queen, who mistakes her for Amadeus, whom she intends to seduce. Maria Anna stalls for time by playing a captivating piano piece until the Emperor finally intervenes and expels his sister from the court. The distraught Queen reveals the true reason for her return: Her husband, King Louis XVI, demands an heir after years of childlessness – fathered by a man of his choosing. But she has chosen Amadeus!
King Louis XVI arrives in Vienna to bring Marie Antoinette back. Determined to take control of her own destiny, she seduces Amadeus and begs him to come to her court in France. After he rejects her, the queen turns to Salieri and offers him a pact to sabotage his rival once and for all. Upon his return to Vienna, Amadeus is shocked to learn that Maria Anna, with the support of Leopold and Constanze, is running the opera.
Alone and filled with remorse, Amadeus rejects the queen and begs Constance for forgiveness. He vows to bring Maria Anna back. Marie Antoinette, having no other choice, returns to France. Maria Anna's fiancé, Baron von Sonnenburg, recognizes her talent at their first meeting and agrees to break off the engagement. After an emotional conversation between Maria Anna and Amadeus, they free themselves from Leopold's control and vow to trust each other again.
