Anna Maria and Wolfgang set out for Mannheim, unaware that Colloredo has already sent word ahead to the Prince to assure that Wolfgang will not find work with him. The show moves ahead to an inn in Mannheim ("La chanson de l'aubergiste"), where Wolfgang enters with several finished commissions. After sharing his dream of writing a grand opera in German, he is ridiculed by several of the people in the bar who remind him that all operas are written in Italian. Wolfgang rejects their comments ("Le Trublion") and a fight breaks out, during which both he and his mother flee before being arrested by the police. A local copyist meets them, already aware of Wolfgang's musical prestige, and offers to copy all of his compositions free and help him distribute them for a small condition – coming to visit his family. The copyist is Fridolin Weber—Cecilia's husband—and they intend to have Wolfgang help them prepare their daughter, Aloysia, for a ball for the Princess of Orange to boost her fame. Anna Maria and Wolfgang visit the Webers at their home, where Wolfgang is introduced to Sophie, Josepha, and Constanze. Upon the appearance of Aloysia ("Bim Bam Boum"), Wolfgang falls head over heels and forgoes his other commissions to help her prepare for her showcase to the Princess. He and the other members of the Weber family are unaware that Constanze has already started to fall in love with the young composer ("Ah! Vous dirais-je maman"). Anna Maria, seeing that her son is prepared to throw away his musical career over his love of Aloysia, writes home to Leopold in the hopes that he will be able to snap Wolfgang out of his enamored state. She attempts to do this by telling Wolfgang that his commissions have been dropped and he's been rejected for employment, but Wolfgang insists that Aloysia is his future and displays no worry. At the ball, Wolfgang and Aloysia display their respective musical talents, while Fridolin and Cecila attempt to get the Princess of Orange to take an interest in their daughter. While Wolfgang discusses music with the Princess, Aloysia discovers Constanze's jealousy over her relationship with Wolfgang ("Six pieds sous terre"). Wolfgang returns to announce that Aloysia will, indeed, be employed with the Princess of Orange and seems prepared to dedicate his life to her when Anna Maria arrives with a letter from Leopold ("J'accuse mon père").