The Temple of Juno, Queen of the Gods, wife of Jupiter. Semele, daughter of Cadmus, is engaged to marry Athamas, and the family gathers to celebrate the nuptials. Not everyone is happy, however; Semele's sister, Ino, secretly loves Athamas. And Semele herself is really in love with the god Jupiter, also known as Jove. She prays to him to intervene, whereupon the Mighty Thunderer breaks up the wedding with dreadful omens.
Privately, Ino admits that she loves Athamas. Cadmus interrupts them to say that Semele was abducted and whisked away to the realm of the gods. There Semele discovers a world of "Endless pleasure, endless love."
Juno's Dwelling. The goddess Iris tells Juno (who is also known as "Saturnia") about the gorgeous pleasure-palace Jupiter has created for Semele. Juno vows to destroy her new rival. In the aria "Hence, Iris, away," she and Iris set out on their quest: Juno plans to enlist the help of Somnus, god of sleep.