Cadmus and his attendant priests rejoice that the goddess of marriage has favored the union of Semele and Athamas. But the unhappy girl, dreaming of her love for Jupiter, implores the god to help her to defy or to accept her parents’ will. Although tormented by her own unspoken love for Athamas, lno dares not confess her feelings to the others. When the flames on Juno’s altar are suddenly extinguished, the priests grow fearful of the gods’ displeasure. As the fire rekindles, only to subside again, they are certain that the marriage has provoked the wrath of Jupiter.

All but Athamas and lno depart in terror. Gradually, Ino makes the prince understand the true meaning of her sorrow. Despite his disappointed hope of marriage to her sister, Athamas is deeply moved by lno’s love for him. Cadmus returns to tell them that Semele has been borne skyward by a giant eagle – a miraculous event which the priests interpret as a sign of godly favor. Appearing in a vision, Semele rejoices that, as Jupiter’s beloved, she has begun a new life of ecstasy.

Jupiter has installed Semele in a splendid palace. Stung by her husband’s latest infidelity, Juno swears vengeance on her rival. Iris, messenger of the gods, cautions her that the palace is protected by a pair of fire-breathing dragons. But Juno says she will appeal to Somnus, god of sleep, to close their eyes.

Awakening in her palace, Semele longs to sleep again, since dreaming reunites her with the absent Jupiter. He soon appears, assuring her that although he comes to her in mortal form, he will never deceive her as might a human lover. But Semele’s happiness is incomplete, for she remains a mortal -and perhaps unworthy of a god’s devotion. Jupiter, uneasy at her longing for divinity, seeks to divert her with human companionship by summoning her sister Ino. He transforms the palace into a landscape of Arcadian delights, and withdraws as the sisters greet each other and marvel at the enchantments all around them.

Juno and Iris call up Somnus from his slumber. Somnus must set Jupiter to dreaming about his mistress and so inflame him that, when he wakes, desire will have rendered him unable to refuse her anything. Further, Somnus must lend Juno his leaden staff to lull to sleep the dragons, and then cast a spell on Ino – allowing her to take on the girl’s appearance and so gain access to the palace.

So disguised, Juno brings Semele a magic mirror. Gazing in the glass, Semele marvels at her own beauty. Having encouraged the girl’s ambition, Juno tells her to withhold her favors from Jupiter until he agrees to love her in his godly form – and thus elevate her to the rank of goddess. Juno leaves, confident that she has sent her rival to her doom.

Semele complains that her readiness to yield to Jupiter’s every whim goes unreciprocated. When Jupiter agrees to grant whatever she may wish, she asks that he appear to her in his full glory as the ruler of the gods. Realizing that he cannot avert the catastrophe Semele has wished upon them, the unhappy god departs.

Awaiting his return, Semele begins to regret her folly, but all too late: as the god descends upon the palace in a storm cloud flashing bolts of lightning, she is consumed by fire.

The Thebans mourn her tragic end, proof that mortals should not aspire to overreach their human destiny. Having learned that the gods smile upon her love for Athamas, Ino is betrothed to the prince. Apollo appears and prophesies that a new deity – Semele’s son by Jupiter – will rise in glory from his mother’s ashes. The Thebans hail Bacchus, whose blessings on mankind promise to surpass the joys of love itself.