Act I:

On February 29, 1880 (maybe!), a gang of Pirates led by their “King” celebrate the birthday of their apprentice Frederic on the coast of Cornwall, England.  He reminds them that his obligation is finished, since he was apprenticed only until his 21^st birthday.  The pirate maid Ruth, who had been Frederic’s nursery maid, explains that the boy should never have been left with them: thanks to her failing ears, she had placed Frederic with a pirate, instead of a pilot!  Frederic admits he is a “slave of duty” and that when he quits them he intends to destroy them, not because he doesn’t love them, but because he loathes what they do.  Frederic further explains that their lack of pirating success is due to their soft-heartedness: having sworn never to victimize orphans, everyone they catch, of course, claims to be one.  Ruth is determined to stay with Frederic, even to wed him.  Since he has spent his entire life with the Pirates, completely deprived of knowing any other woman, the innocent—and gullible—boy accepts Ruth despite their difference in age.  However, just as Frederic promises his hand, he hears a chorus of girls nearby.  Upon seeing their beauty, his understanding of female pulchritude is shattered, and he realizes that Ruth has not been completely forthcoming about her own “assets.”  Frederic drops Ruth, the girls enter, and Frederic confesses his profession.  The girls are terribly intimated by the strapping boy…all but Mabel, who falls in love with Frederic instantly.  Pirates sneak up onto the beach and grab a girl.  Major-General Stanley enters, identifies himself as the girls’ father, and the Pirate King tells him that they have eyes for his daughters.  Stanley objects and, knowing the Pirates’ reputation, claims he is an orphan, thus, disarming them.  Accompanied by Frederic he escorts his daughters back to the family estate.  The disappointed Pirates—and a frustrated Ruth—are left behind.

Act II:

The Major-General (who is actually no orphan) sits alone in the ruined chapel of Tremorden Castle, regretting the lie he had earlier told the Pirates.  Meanwhile, Frederic hatches a plan to lead a squad of inept Police against his old gang.  Before he can act, however, the Pirate King and Ruth arrive to tell him that he is still obligated to the Pirates:  Because Frederic was born on February 29 of a leap year, he has technically served only five years, not the 21 required by his contract.  A strong sense of duty forces Frederic to relent, and, because he is once again a member of the Pirates, he must also reveal the truth that Major-General Stanley is not an orphan.  The Pirate King vows that he will have revenge on the Major-General for this shameful duplicity.  Mabel begs Frederic not to return to the Pirates, but he reluctantly and dutifully ignores her pleas.  The Police try to bolster their sagging courage about attacking the Pirates, who ultimately manage to capture the Major-General, his daughters, and even the pitiful Police.  However, when Ruth divulges that the Pirates are, in fact, noblemen who swear their allegiance to the Queen, the tables are turned—and the Pirates are compelled to surrender to the Police.  Because the Pirates are loyal—and royal—British subjects, they are forgiven.  The Major-General’s daughters take their pick of ex-Pirates and Police, Frederic wins Mabel, Ruth gets her own man, and everyone lives happily ever after!