The opera is set in Venice in 1727. In the house of Marcello, a musician and poet, two of his pupils Rosana and Faustina wait for their teacher who is late for their singing lesson. Marcello at last arrives, apologises, and announces that instead of their lessons
he wants them to sing for a guest of his, the famous composer Adolph Hasse, from Germany. Rosana sings first: a sad song about unrequited love. Hasse praises her warmly for singing from the heart, whereas Marcello is just puzzled by her sudden ability to find
the correct tone. Faustina immediately recognises that Rosana is singing about her love for Marcello, and is irritated that he cannot see it himself. She then sings a joyful song about a nightingale, which leaves both Hasse and Marcello captivated.Hasse expresses his profound admiration for Marcello and she invites him to accompany her to a party that evening.
Faustina is preparing for the evening and her thoughts turn to Hasse, and she realises that she loves him. Hasse has quietly arrived and, overhearing her soliloquy, immediately tells Faustina that he has fallen in love with her too. Marcello, masked, arrives and
stands outside Faustina’s rooms, consumed with love for her and intending to sing her a serenade. He hears the gondola bearing Faustina and Hasse approaching, and jealously listens to their loving conversation.
Hiding while they enter her apartment, he vows revenge on Hasse. Marcello wants to pick a quarrel and both men draw their swords. Faustina, who has heard the commotion outside, opens her balcony window, sees the events below and collapses with a
scream. Hasse leaves, telling Marcello that he may keep his life as a pledge of their friendship.
In Marcello’s reception room, Rosana spots a newly written song by him, she sings it with growing feeling. It is about the desolation of a spurned lover, and she realises that Faustina has rejected Marcello, but recognises with sadness that she herself is still nothing to him. Marcello enters and announces to a shocked Rosana that he is leaving Venice, and will move to Brescia as its governor. Rosana is adamant that she will not abandon him and will go wherever he goes and to her joy, he agrees that she can come with him. Hasse announces that he and Faustina are to marry and will leave for Germany. Faustina asks for Marcello’s forgiveness, and emphasises her gratitude for all that he has done for her. He tells his visitors that, as Brescia’s governor, he will have little time for music, but that Rosana will go with him. Faustina is delighted, and confides to him that Rosana has always loved him, at which Marcello admits that he never realised how Rosana felt until now. All agree that events have turned out better than they had dared hope, and the two couples part happily.