At the opening of the opera, they recall how they met. Louise's mother interrupts them and vocally expresses her disapproval of Julien. The exhausted father comes home from work and his wife and daughter implore him to quit the taxing job. However, he feels that it is his responsibility to provide for his family. At supper, he reads a letter that Julien left in which he proposed marriage to Louise. He is indifferent, but the mother is livid and, when Louise stands up for Julien, she slaps Louise across the face. The peaceful father asks his daughter to sit with him and read the paper. As she reads about springtime in Paris, she breaks down and cries. "Scene 1: A street in Paris"  It begins with a prelude that suggests dawn in Paris. The curtain rises to a bustling scene where people go about their daily routines and comment about life in general. The Noctambulist enters and calls himself the spirit of the Pleasure of Paris, and then leaves with the daughter of a ragman. Julien appears with a group of fellow bohemians to show them where Louise works. He tells them that if her parents do not consent to marriage, he will carry the girl off. Julien and his companions go off and he sings that the medley of sounds around him is the voice of Paris itself. Louise and her Mother arrive at the dressmaking store where Louise works (her mother brings her to work everyday). When the mother leaves, Julien returns. Louise tells him she loves him, but she loves her parents too much to leave them. He tries to persuade her to run off with him and she finally agrees to do so soon. "Scene 2: Inside Louise's place of work"  Louise is being teased by the other seamstresses for being in love. A band is heard outside and Julien sings a serenade. The girls admire him for his looks and voice. Louise quietly slips away – to run off with Julien. "A cottage overlooking Paris"  The act opens with the opera's most well known aria, "Depuis le jour"; the lovers have moved into a cottage overlooking Paris and in the aria she sings of her happiness with her new existence and with her lover. A long love duet ensues in which they sing of their love for each other and Paris. Many Bohemians enter and crown Louise Queen of Montmartre. The Noctambulist presides as the King of the Fools.