A poorly furnished room in a rich man's house in Vienna
A stage is being constructed for the performance which is to take place that evening.
The music master has just heard that the opera seria Ariadne by his composer pupil is to be followed by an opera buffa, but his protests to the major domo are swept aside with the reminder that the wealthy patron can arrange matters as he pleases.
The composer has a confrontation with an insolent lackey when he wants to give instructions to his musicians and discovers they are playing dinner music for the guests, but his indignation vanishes as a musical inspiration strikes him. He is not even disturbed by a brawl between the tenor and the wig maker. The dancing master, in charge of the commedia dell'arte troupe whose production is to follow the opera seria, explains the situation to Zerbinetta. The composer is struck by her beauty but horrified when he learns that her performance is to follow his opera. The music master tries in vain to console him.
The dancing master explains to Zerbinetta the advantages of her show following the opera, while the music master tries to soothe the prima donna by telling her that only Ariadne will be remembered the next day. The major domo announces that his master has decided that the two works must be played simultaneously, so that a fireworks display can start punctually at nine o'clock. Besides, it has occurred to his master that a scene so stark as a desert island would be out of keeping with his sumptuous mansion, and he wishes its bareness to be relieved by the addition of the commedia dell'arte figures.
The music master restrains the composer from leaving by reminding him that he needs the fee to live on, and the music master and dancing master put their heads together to devise a solution, the latter explaining that Zerbinetta, as a mistress of improvisation, will be able to fit in with the opera and to give her companions the lead. The prima donna approaches the music master while the tenor whispers to the composer, each trying to keep his part intact at the expense of the other.
The dancing master tells Zerbinetta the plot of the opera - that Ariadne is a princess deserted by her lover and longing for death. Zerbinetta's interpretation - that Ariadne is only waiting for another lover - stirs the composer into trying to explain that Ariadne is a high-souled being who can only love one man; but when she learns that Ariadne is rescued by Bacchus, she declares her point proved and assures the composer he will soon know more about women. She then tells her companions her version of the story, in which they are travellers who have landed on Ariadne's desert island and must cheer her up when the chance occurs.
Then she turns her charms on the composer and he is smitten, standing in a daze when she leaves. The music master convinces the prima donna that the best way she can demonstrate her superiority to Zerbinetta is not to refuse to take part but to show her up on the stage. The composer is rhapsodising about the beauty and power of music when the appearance of the commedia dell'arte troupe about to go on stage brings him abruptly back to earth.
A desert island
Ariadne lies sleeping at the mouth of a cave, watched by Naiad, Dryad and Echo who comment on her ceaseless lamenting. She wakes and continues to grieve, watched sympathetically from the wings by Zerbinetta and her companions. The nymphs try to rouse her, but she refuses to stir and declares her intention of waiting for death.
Zerbinetta and her companions make a vain attempt to attract her attention and Harlequin sings about the passing of sorrow and the return of love; but she does not even lift her head. She continues to long for the purity of the land of death.
As Harlequin, Brighella, Scaramuccio and Truffaldino sing and dance in front of Ariadne, Zerbinetta reflects that although she herself would have no trouble finding one of them attractive, Ariadne is unmoved. So she dismisses them and tries to explain to Ariadne that although they are of different stations in life, yet both are women with a common fate. Others, too, have been abandoned; even she has cursed the faithlessness of men; but then, women too are not above changing their hearts. Every time she thinks she will be constant to a lover, a new love tempts her and she falls. Each one is welcomed like a god, but this does not stop her from succumbing to a new god.
Harlequin admires her sermon but comments that she has wasted it on deaf ears and proceeds to woo her. She evades him and flirts with the other three, who are also courting her. She flits from one to the other, then suddenly leaves them and runs off with Harlequin.
The nymphs run in with news for Ariadne of the arrival of a god. They relate his upbringing by nymphs, his arrival at the island of the enchantress Circe, her attempt to enslave him and his escape. Bacchus appears, rejoicing in his freedom from Circe, and Ariadne, hearing his voice, thinks he is the messenger of death, a delusion she continues to labor under when they stand face to face. Bacchus is struck by her beauty and asks if she is the goddess of the island and is she, too, a sorceress with the power to transform men. She greets him expecting death, but he assures her that only now is life beginning for both. She faints and thinks she has died, astonished by the magical transformation which seems to have overtaken her, while Bacchus exclaims that hers is the magic which has transformed him. They do not heed Zerbinetta who pops back to remind the audience that the coming of a new god was how she had described the coming of a new lover.