Act 1. Cadmus prepares for the marriage of his daughter Semele to Athamas but she has been inventing any possible obstacle to the arrangement, as she is in love with the god Jupiter. Semele’s sister, Ino, reveals her secret love for Athamas. Cadmus hears that Semele has been mysteriously whisked away to unknown realms. Semele revels in her new-found pleasures.
Act 2. Juno, goddess of marriage and children, is furious at her husband Jupiter’s infidelity and discovers from her helper, Iris, that Jupiter has built a pleasure palace for Semele. Juno swears vengeance on Semele, and decides to enlist the help of Somnus, god of sleep.
Semele longs for Jupiter’s return. He arrives and reassures her of his love, but realises that she has dangerous ambitions of immortality. In order to distract her he uses his godly powers to pacify her, and summons Ino to keep her company.
Act 3. Juno and Iris manage to rouse Somnus from his slumber. Juno orders Somnus to give Jupiter a dream that will consume him with such a passion for Semele that he will do whatever she asks.
Disguised as Ino, Juno gives Semele a mirror that makes her think she is the most beautiful creatures in existence. She convinces Semele that she must refuse to return Jupiter’s love until he agrees to give her whatever she wants. She should insist that he comes to her bed not as a mortal but in his full godly glory. Semele agrees.
Jupiter is so desperate for Semele that he agrees to her conditions but is horrified by her demands as he knows that giving in to them will result in her death.
Juno exults in her victory. Semele realises the folly of what she has done – too late.
Ino is horrified by Semele’s fate. Apollo appears to announce that Semele has left a legacy for the world: Bacchus, god of wine.