Cadmus, a mortal King, and his followers have traveled to the Temple of Juno, Queen of the gods, to bless the marriage of his daughter Semele to Prince Athamas. A sacred fire develops on the altar and the chorus proclaims that the omens for the marriage appear favorable. Semele arrives for the ceremony after many attempts to delay the marriage due to her love of Jupiter, King of the gods. She pleads to Jupiter to intervene; his responding thunder interrupts the ceremony. The symbolic flames on the altar of Juno are extinguished as Cadmus prays for their return. Sensing a war between the godly spouses, the party flees from the temple, but Athamas and Semele’s sister Ino remain.
Ino and Athamas each reveal surprising truths and are startled when Cadmus interrupts with the shocking news that Semele has been abducted by Jupiter. The trio languishes while Semele transcends to the realm of the gods.
Juno, angered at her husband’s adultery, has ordered her messenger Iris to discover where Jupiter has taken Semele. Iris reports that Jupiter has built his new mortal lover an elaborate palace guarded by fierce dragons that never sleep. The enraged Juno swears vengeance, and hurries to visit Somnus, the God of Sleep, to demand his aid.
Semele wakes and immediately calls for Jupiter. He arrives in human form, reassures her of his fidelity and reminds her of her fragile mortality. Semele professes devotion to him, but reveals her discontent that she has not been made immortal. Sensing Semele’s dangerous ambition, Jupiter magically summons her sister Ino from earth to keep her company. In preparation, Jupiter transforms the palace into a beautiful garden to comfort and distract her…
Ino, enraptured, describes the extraordinary journey which lead her to Mount Cithaeron. The sisters are joyfully reunited, but Semele is once again distracted by the pursuit of pleasure and love.
Somnus is disturbed from his deep sleep by the arrival of Juno and Iris. He wakes but refuses to help Juno. She offers his favorite nymph in return for his aid and he relents. Juno orders Somnus to give Jupiter an erotic dream that will make him desperate for Semele at any price. Juno then receives Somnus’ potent power in order to lull any being to sleep on command. She uses that power to manipulate Ino, whom Juno puppets in order to convince Semele that she has been made as beautiful as a god.
Trusting her sister, Semele sees her glorified reflection and becomes narcissistically obsessed. Juno, still disguised as Ino, advises that if Semele wishes to become truly immortal then she must refuse Jupiter until he promises to grant any wish she desires. The disguised Juno suggests that Semele demand Jupiter to come to her only in his true godly form. Semele eagerly accepts this advice. Juno departs when she senses the approach of her husband.
Jupiter is alarmed when Semele rejects him. He rashly swears an irrevocable vow to grant her whatever she desires, and she demands that he visit her in his true form. He reacts with horror, knowing that his non-human form will instantly kill her. Semele refuses to listen, assuming that Jupiter’s protestations are only to resist granting her immortality. Left alone, Jupiter tries to find a way to save Semele’s life, but is forced to accept his loss.
Semele sees Jupiter approach as a fiery cloud of lightning and thunder, laments her folly, and dies consumed in flames.
Jupiter descends with the pleased Juno and announces that from Semele’s ashes the unborn child of Semele and Jupiter has risen. Named Bacchus, God of Wine, he will bring a reckless delight to all the earth.