A shepherdess asks the seer whether she will be happy in love (Air: "Je ne sais quel ennui me presse") and he replies in the affirmative. Tirésie begins to interpret the song of the birds: he tells his listeners to beware of the wrath of the god of the sea and that Naïs will find true love. Astérion and Télénus decide that the oracle means they must sacrifice the life of their rival to appease Neptune (Chorus: "Aux armes, vengeons-nous").
In this narrative, a young shepherdess consults a prophet about her romantic prospects (Air: "I am not aware of what kind of sadness is weighing upon me") and he reassures her that she will be happy. Tirésie begins to interpret the birdsong, warning his audience to beware of Neptune's wrath and predicting that Naïs will find true love. Astérion and Télénus decide that the oracle implies they must sacrifice their rival's life to placate Neptune (Chorus: "To arms, avenge ourselves").