Calchas, the High Priest prophesies that the goddess Diana has demanded that king Agamemnon sacrifice his daughter, Iphigénie, in order to guarantee fair winds for the king's fleet en route to Troy. Throughout the opera, Agamemnon struggles with the terrible choice between sparing his daughter's life and ensuring his subjects' welfare. Agamemnon summons his daughter to Aulis, the port where the Greek navy is gathering, ostensibly for her to wed Achilles, the great warrior hero. Then, doubting his decision, the king sends Arcas, his captain of the guards, to send her back home with the fabricated explanation that Achilles has been unfaithful. However, Arcas also has second thoughts, and he ultimately reveals the plot to Achilles. Achilles rushes to save Iphigénie from sacrifice; however, the Greek people have taken Calchas' prophecy to heart, and they demand the ceremony go forward. At this point, Iphigénie resigns to her fate, and offers her own life for the sake of her people. Just in time, however, Calchas announces that Diana has changed her mind about the sacrifice and Iphigénie’s life is spared.