Act 1
Cadmus, a powerful patriarch, is preparing for the marriage of his daughter Semele to Athamas, scion of another rich family. Signs from Juno, the Goddess of Marriage, indicate she approves of the match (Chorus: Lucky omens bless our rites.)
However, the bride has been inventing one excuse after the other to put off the wedding and her father and would-be bridegroom urge her to hesitate no longer (Duet: Daughter, hear! Hear and obey!)
To herself, Semele reflects on her dilemma – she does not wish to marry Athamas as she is in love with Jove himself, the King of the Gods, and calls on him to assist her (Accompanied recitative: Ah me! What refuge now is left me; arioso: O Jove! In pity teach me which to choose).
Ino, Semele's sister, expresses her own state of distress as she is in love with Athamas (Quartet: Why dost thou thus untimely grieve?)
Jove has heard Semele's prayer and his thunderbolts interrupt the proceedings and alarm the observers (Chorus: Cease, cease your vows), which all do except for Athamas, in despair at his wedding being cancelled, and Ino, hopelessly in love with him (Aria: Turn, hopeless lover).
Athamas is astonished when she tells him bluntly that she loves him (Duet: You've undone me).
Cadmus interrupts their confusion and describes the extraordinary event he has just witnessed: as they fled the temple Semele was suddenly carried off by an eagle (Accompanied recitative: Wing'd with our fears).
As the act ends, Semele is seen enjoying her role as Jove’s new mistress (Aria: Endless pleasure, endless love).
Act 2
Scene One
Iris, Goddess of the Rainbow, arrives in a beautiful pastoral landscape and is soon joined by Juno, spurned wife of Jove and Queen of the Gods. Suspicious of her husband's conduct, Juno has sent Iris to find out what she can. Iris reports that Jove has installed Semele as his mistress in a palace atop a mountain (Recitative: Iris, impatient of thy stay)
The outraged Juno swears to have revenge (Accompanied recitative: Awake, Saturnia, from thy lethargy!) and then decides that she and Iris will pay a visit to the god of sleep in his cave, in order to get magical assistance for her plan to win Jove back (Aria: Hence, Iris, hence away).
Scene Two
An apartment in the palace of Semele.
Semele awakes and regrets that the dream she was having of being with her lover has ended (Aria: O sleep, why dost thou leave me?).
When Jove enters, in the form of a young man, she tells him how difficult it is for her when he is absent. He explains that she is a mortal, unlike him, and needs to rest from their love-making from time to time. He attempts to assure her of his fidelity (Aria: Lay your doubts and fears aside).
Semele, however, is beginning to be unhappy that her lover is a god and she a mere mortal. This sign of an ambition to immortality in Semele worries Jove who decides he must distract her from such thoughts (Aria: I must with speed amuse her).
Jove has arranged for Semele's sister Ino to be magically transported to the palace, to keep her company, and promises that the gardens and environs will be paradise (Aria: Where'er you walk).
He leaves, and Ino appears, describing the wondrous experience of being flown there by winged zephyrs (Aria: But hark! The heav’nly sphere turns round).
The sisters sing of the joy they are experiencing, hearing the music of the spheres (Duet: Prepare then, ye immortal choir) and a celestial choir declares that this part of the earth has become a heaven (Chorus: Bless the glad earth).
Act 3
Scene One
The Cave of Sleep. Somnus, the God of Sleep, lying on his bed.
Juno and Iris arrive and wake Somnus (Accompanied recitative: Somnus, awake), to his displeasure (Aria: Leave me, loathsome light).
He only gets out of bed when he hears Juno mention the beautiful nymph Pasithea (Aria: More sweet is that name).
Juno promises he will have the nymph if he will lend her his magical aid. She tells him to put to sleep the dragons that guard the palace where Semele is ensconced as her husband's mistress, and transform Juno herself into the likeness of Semele's sister Ino. Somnus agrees (Duet: Obey my will).
Scene Two
Semele's Apartment.
Juno, now in the form of Ino, enters and feigns astonishment at Semele's increased beauty. She exclaims that Semele must have become a goddess herself and gives her a mirror (Recitative: Behold in this mirror).
Semele is enraptured by her own beauty (Aria: Myself I shall adore). "Ino" advises Semele to insist that Jove appear to her in his real, godlike form, and that will make her immortal herself (Accompanied recitative: Conjure him by his oath).
"Ino" leaves and Jove enters, eager to enjoy Semele (Aria: Come to my arms, my lovely fair) but she puts him off (Aria: I ever am granting).
He swears to give her whatever she desires (Accompanied recitative: By that tremendous flood, I swear) and she makes him promise to appear to her in his godlike form (Accompanied recitative: Then cast off this human shape).
He is alarmed and says that would harm her (Aria: Ah, take heed what you press), but she insists he keep his oath (Aria: No, no, I'll take no less) and leaves.
Jove knows this will mean her destruction and mourns her impending doom (Accompanied recitative: Ah, whither is she gone!).
Juno triumphs in the success of her scheme (Aria: Above measure is the pleasure).
Scene Three
Semele, granted her wish to see Jove in his true godlike form, is consumed by his thunderbolts, and as she dies she regrets her own foolishness and ambition (Accompanied recitative: Ah me! Too late I now repent).
Watching this, her family express their amazement (Chorus: Oh, terror and astonishment!). Ino then reveals that she has been told by the gods that she should marry her beloved Athamus, who readily agrees to this proposition. All celebrate the fortunate outcome (Chorus: Happy, happy shall we be)