The returning Frederick promptly informs the outlaws of the deception of Mabel's father, leading to a showdown with the police, after which it is revealed they are all "noblemen who have all gone wrong."  
The title characters engage in a scheme of abduction and matrimony, proclaiming "here's a first-rate opportunity" upon meeting a gaggle of girls.  The girls are also the objects of interest to the protagonist, who begs them to help him regain a respectable life, asking if "any maiden here, whose ugly face and bad complexion, has caused all hope to disappear, of ever winning Man's affection."  Later, however, the protagonist learns that he was mistaken about his future: he was apprentice until his 21st birthday, not his 21 year, and since he was born on February (*) 29th of a leap year, his apprenticeship will expire only when he is over 80 years old.  The returning Frederick promptly informs the outlaws of the deception of Mabel's father, leading to a showdown with the police, after which it is revealed they are all "noblemen who have all gone wrong."  FTP -- name this satirical Gilbert and Sullivan opera, whose most famous musical number is "I am the very model of a modern major-general."
The title characters engage in a scheme of abduction and matrimony, proclaiming "here's a first-rate opportunity" upon meeting a gaggle of girls.  
The {Pirates of Penzance} or {Slave} to {Duty}