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.