The setting is Paris in 1840. Scene 1 - an artist's garret, through which the snowy rooftops of the city are visible. The young composer Roberto Randel is at the piano, working on a fragment of a romantic opera which he is writing with his friend Victor, and which is to become the leitmotif of the zarzuela (Romanza y Dúo: "Mudos testigos de mis amores".) The librettist joins his friend at the piano, but they are soon disturbed by the voice of their neighbour's daughter, Cossette, outside in the street (Canción: "La mariposa, de rosa en rosa".) Roberto complains that she is always singing his songs out loud, which stops him working. Victor tells him not to mind too much, as she has a pleasant voice and is exceedingly pretty. Marcelo, Cossette's father, joins them. He asks to borrow Roberto's coat - the only one they have between them - so he can accompany his daughter in the open audition that evening at the Opéra-Comique. If Cossette is successful, it will put the whole family on a sounder financial footing. All three leave for the Bola de Oro Café, hoping to wangle a meal without paying, and leaving their key with the caretaker Pelagia.

Cossette puts her head round the door to check that Roberto is out, and joins Pelagia, who has stayed to tidy up (Dúo: "La niña de ojos azules".) The older woman teases her - why be so backward in coming forward? Cossette responds by admitting that though she does indeed yearn for Roberto, she must put her musical ambitions first. After that, there will be time for love (Romanza: "Si es amor el sentir".) While Pelagia goes to fetch wood for the stove, Cossette purloins Roberto's score, intending to sing it at the audition that evening. Another neighbour, Girard, who is forever promising to help everyone, appears. True to form, he says he will help her clinch the job that evening, and the two of them leave an invitation for Roberto on the keyboard. After they leave two shopgirls, Juana and Cecilia, come in and leave a badly-spelled note for Roberto, asking him to dine with them at their friend Mimi's. When he returns he discovers both notes, deciding to have dinner with Mimi before going on to the Opéra-Comique.

Scene 2. A square in the Quartier Latin, close to the Bola de Oro and a dancehall. Couples are heading for the dance, and the sound of a delicate waltz drifts through the night air. Victor emerges from the café having been unable to get free food out of the landlord. He tries to flirt with Cossette, who comes looking for her father in the restaurant before going off to sing, but she ignores him (Escena: "La noche misteriosa".) Temporarily dashed, he lurks in the background when a party of his friends burst in. They sing the intense Coro de Bohemios: "En la luz del sol que enciende" in praise of passion and the bohemian way of life. Victor sees the two shopgirls heading for the dance and tries his luck with them, but the well-heeled Girard whisks them off for drinks in the café. Finally Victor admits defeat, and meeting Marcelo goes off with him to get a meal.

Cossette, coming out of the restaurant, nearly runs into Roberto, who is in high spirits after his dinner with Mimi. She hides to avoid him, but becomes furious when she sees him chatting up the shopgirls when they reappear from the café (Cuarteto: "¡Qué alegre es el cielo!".) Taking her courage in both hands she calls to him, and he immediately diverts his attention to the pretty newcomer. The two girls leave, with pert curtseys to their successful rival, and a romantic scene ensues (Dúo:"¿por qué vuestros labios?".) When Roberto asks for her name Cossette tells him, but she is torn between accepting his amorous overtures and thoughts of her career. He leaves her, and the scene ends with Cossette, Girard and Marcelo heading off for the Opéra-Comique, together with the luckless and still unfed Victor. The celebrated intermezzo (Intermedio) leads into the last scene.

Scene 3. A lavish party in the artists' salon of the Opéra-Comique. Girard is telling Victor that his libretto is too good for Roberto, and should be sent to someone important like Auber, whom he says he knows. Victor heads off to buttonhole the famous composer. Girard reassures Cossette and Marcelo that all will be well, and then tells Roberto that his music is too good for Victor's text - he should be setting someone of the quality of his good friend, Scribe. Roberto goes off to find the famous librettist. Having both been mercilessly snubbed, the young collaborators are furious to discover that Girard has been lying to them. Girard tries to save face by introducing them to one another, and leaves hastily to prepare Cossette for her appearance. Marcelo tells her that she needn't be nervous, as her own father is going to accompany her.

Girard, pretending amidst general bafflement to be Cossette's rich patron, prepares the audience for the appearance of the young singer. Marcelo sits down at the piano but says he is too nervous to read from the score, and Cossette asks for Roberto to accompany her. When he sees his own music on the stand, the composer understands everything. He gladly accepts, and Roberto and Cossette perform the duet from the new opera (Dúo: "Por fin llegaste") to the enraptured audience. The zarzuela ends in a romantic glow, as fame and fortune beckon to all the young bohemians - even the penniless librettist Victor.