Fidelio
Jaquino is a prison guard.  He wants to marry Marzelline, the daughter of Rocco who is the warden of the prison.  But Marzelline has just fallen in love with someone she thinks is a man called Fidelio.  Actually Fidelio is a woman who has dressed up as a man so that she can get into the prison where her husband Florestan is a prisoner (he has been arrested for political reasons).  She is pretending to return Marzelline’s love so that she can find a way of rescuing Florestan.  Rocco says he is happy for Marzelline to marry Fidelio.  Fidelio (Leonore) says she will help Rocco with his job of looking after the prisoners.  Rocco says she can do this, but there is one prisoner she is not allowed to guard.  He has been locked up alone for two years.  Leonore thinks it may be her husband, but she is not sure.