Cadmus, King of Thebes, and his family have traveled to the Temple of Juno in Bœotia to solemnise the marriage of his daughter Semele to Prince Athamus. The Priests and Augurs proclaim that the omens for the marriage seem propitious, but Semele has been inventing one excuse after another to delay the wedding because she is secretly in love with Jupiter. Semele pleads to Jupiter for help, and his thunder interrupts the ceremony and extinguishes the sacrificial flames on the altar of his wife Juno. The Priests advise everyone to flee from the temple, but the despairing Athamus and Semele’s sister Ino remain behind.
Ino reveals to the astonished Athamus that she loves him. Cadmus interrupts with the shocking news that Semele, surrounded by azure flames, has been abducted by a giant eagle, ‘on purple wings descending’, that left behind a scent of ‘celestial odour and ambrosial dew’. The Priests and Augurs identify that this eagle was Jupiter, and Semele is heard to announce that ‘Endless pleasure, endless love, Semele enjoys above.’
Juno, angered at her husband’s adultery, has ordered her messenger Iris to discover where Jupiter and Semele are. Iris reports that Jupiter has built his new mortal lover an elaborate new palace on Mount Cithaeron, and warns that it is guarded by fierce dragons that never sleep. The enraged Juno swears vengeance, and hastens to visit Somnus, the God of Sleep, in order to enlist his aid.
Semele, attended by Loves and Zephyrs, yearns for Jupiter. He arrives, in human form, reassures her of his fidelity, and reminds her that she is only mortal and needs time to rest between their bouts of love-making. Semele professes devotion to him, but reveals her discontent that she has not been made immortal. Jupiter, recognizing that Semele has dangerous ambition, transforms the palace to Arcadia, charms her with its pastoral delights, and magically summons her sister Ino to keep her company. The enraptured Ino describes the heavenly music she has heard on the way to Mount Cithaeron whilst carried by two winged Zephyrs. The sisters, and a chorus of nymphs and swains, sing of the joys of music.
The cavernous dwelling of Somnus, god of sleep, is rudely disturbed by the arrival of Juno and Iris. Somnus lethargically refuses to help Juno, but he is enlivened when Juno promises him the reward of his favourite nymph Pasithea. Juno orders Somnus to give Jupiter an erotic dream that will make him desperate to enjoy Semele’s favours at any price. Juno takes Somnus’ magical lead rod in order to beguile the dragons and Ino to sleep. Juno assumes the form of Ino, pretends to believe that Semele has been made immortal, and gives her a magical mirror that deceives the foolish girl into thinking herself even more beautiful than usual. Juno advises that if Semele wishes to become truly immortal then she must refuse Jupiter’s sexual favours until he promises to grant any wish she desires, and that she must request that he come to her in his true undisguised form (‘like himself, the Mighty Thunderer’). Semele eagerly accepts this advice. Juno departs when she senses the approach of Jupiter.
Inflamed by desire for Semele, Jupiter is astonished when she acts coldly towards him. He rashly swears an irrevocable vow to grant her whatever she desires, and she demands that he visit her in his natural guise. He reacts with horror, knowing that his lightning bolts will certainly kill her, but she refuses to listen to reason, assuming that Jupiter does not wish to grant her immortality. Left alone, Jupiter tries to find a way to save Semele’s life, but dejectedly realizes that ‘she must a victim fall’. Juno gloats in triumph at her victory. Semele sees Jupiter descend as a fiery cloud of lightning and thunder, laments her folly, and dies consumed in flames. Ino, safely returned to Bœotia, announces the tragic news that Semele has perished. However, some good has come of it: Jupiter has ordained that Ino and Athamus must be wed, and Apollo prophesies that Bacchus, god of wine and unborn child of Semele and Jupiter, will arise from Semele’s ashes to bring a delight ‘more mighty than love’ to the earth.