I had sort of a bad feeling about this one.  An opera by a composer
famous for his oratorios, dating from the early days of opera, when
even the best were having trouble with the form.  And to some 
extent this was confirmed.  Music not being among my strong suites,
I tend to judge operas by the dramatic content.  And in the first
act there was essentially none.  There were three choruses, which 
were pretty good.  (Handel apparently just threw them in as a matter
of habit - at least the man knew what he was good at.)

The plot is from mythology.  Semele was one among several mortal 
women who attracted Jupiter's wandering eye, and was set up in a
little love nest on the side.  Juno did not approve, and hatched a
scheme in which Semele was deceived into believing that if she 
persuaded Jupiter to appear in full Godhead, rather than as the
semblance of a man, she would thereby become an immortal herself.  
She teased him into doing so, and she was overwhelmed and destroyed 
by the majesty of the Thunderer.

As I said, the opera is short on drama.  In the second act, the 
goddess Iris tries to save things by overacting outrageously, and
the god Somnus does the same in the third.  But, overall, I would 
have enjoyed the piece about as much sung as a secular oratorio,
rather than as an opera.  (Sacred oratorios were invented by Handel
and others because the Established Church did not permit opera to
be performed during lent.)  I wasn't even much impressed by the 
entrance of Jupiter as a god - the Santa Fe staging is always so
good and so spectacular that I would have expected them to really
go to town on that one, but he just drove on stage in a cart made
of clouds and tin thunderbolts while Semele sank into the stage on 
a descending elevator.