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.
Semele's pleasure-palace. Meanwhile, Semele is troubled (Aria: "O sleep, why dost thou leave me?"). Jupiter has taken human form to be with her, but she is aware that she is a lesser order of being than her divine lover. Jupiter reassures her that he will protect her and always surround her with beauty (Aria: "Where'er you walk"). To distract her, he takes her to a beautiful garden and has her sister Ino brought up from earth to keep her company. Together the two sisters experience heavenly bliss.
The cave of Somnus. Iris and Juno try to rouse the sleepy Somnus, who responds with the aria "Leave me, loathsome light." But when Juno promises him the nymph of his dreams, Pasithea, Somnus agrees to help Juno. Somnus will weaken Jupiter with uncontrollable lust and lock in sleep both Ino and the dragons that guard Semele's palace.
Semele's palace. Disguised as Ino, Juno gives Semele a magic mirror, which makes her fall in love with herself (Aria: "Myself I shall adore"). Juno-as-Ino tells Semele that if Jupiter makes love to her in his divine form, instead of his mortal disguise, Semele herself will become a goddess. She urges her "sister" not to waste this opportunity, and Semele takes the bait.
When Jupiter returns, Semele makes him swear he'll give her anything she desires. Then she demands that he appear to her in his divine form. Jupiter is horrified by the request: "Should I grant your request, I shall harm you." But there is no dissuading her (Aria: "No, no, I’ll take no less / Than all in full excess!"). Reluctantly, Jupiter appears in his true form, and Semele is consumed by flames.
Back on earth, everyone is terrified by Semele's fate. Ino unites with Athamas, and Apollo announces that from Semele's ashes, Bacchus, the god of wine, will be born.