The Nymph of the Seine longs for Louis XIV to return from battle. A Soprano assures her that he follows Glory and will return. A celebration of the pastoral divinities commences. At the wedding of Admetus and Alcestis. Under the guise of a party, Lycomedes abducts Alcestis. A following storm, perpetrated by his sister, the sea nymph Thetis, helps his escape.
Atys sings the air "Allons, allons" at a brisk tempo to wake up the Phrygians so that they can give a proper welcome to Cybèle. Idas mocks him suggesting his motivation might be too much love for the goddess in the air "Vous veillez lorsque tout sommeil." Sangaride and Doris arrive and Sangaride is betrothed to the king of the Phrygians, Celenus, and pretends to be excited for the wedding, especially because the goddess Cybèle will attend. We learn that Sangaride is in fact unhappy about her forthcoming wedding, for she is really in love with Atys. Atys finds Sangaride lamenting and confesses his love for her in the and Sangaride is astonished by his declaration of love.
The prologue, in praise of King Louis XIV, represents him as Apollo slaying the Python of Delphi. The opera itself concerns the love story of Cadmus, legendary founder and king of Thebes, Greece, and Hermione, daughter of Venus and Mars. Other characters include Pallas Athene, Cupid, Juno, and Jupiter.
The plot of the tragedy of Isis is loosely adapted from one of the episodes in Ovid's Metamorphoses. Its plot parallels that of Lully's previous opera, Atys (in which Sangaride, promised to Celoenus, is pursued by another and acquires a goddess as a rival). 'Isis centers around the god Jupiter's love for the nymph Io and the jealousy of Juno. Io, daughter of Inachus, is promised in marriage to Hierax, but is pursued by Jupiter, and yields to this love in spite of her feelings of guilt. Juno has Io imprisoned and tortured, leading Io to cry out to Jupiter for help. He swears faithfulness to Juno if she will spare Io, and Juno turns Io into a goddess: Isis, the Egyptian goddess.
The plot centres around a love triangle: Egée wants to marry his ward, princess Églé, while the sorceress Médée wishes to marry the young warrior Thésée, but Thésée and Églé love each other. Médée attempts to force the lovers to renounce each other: first by using her magic to bring Églé to a place of torment, then by convincing Egée to have Thésée killed as a potential threat to his reign. But before Thésée can drink the poison he has been given, Egée realises that Thésée is his lost son. He then gives Églé to Thésée. Médée takes vengeance by destroying the festive setting, but the goddess Minerve undoes this.