When Frederic was yet a little boy, his nurse (Ruth) was told to apprentice him to become a pilot.  She heard the word incorrectly and apprenticed him to band of pirates, remaining with them herself as a maid-of-all work.  Although Frederic loathed the trade to which he had thus been bound, he dutifully served; and as our story begins, his indentures are almost up and he is preparing to leave the band and devote himself to the extermination of piracy.

He urges the pirates to join him in embracing a more lawful calling, but they refuse.  Ruth, however, wishes to become his wife.  Having seen but few women he does not know whether she is really as pretty as she says she is, but he finally consents to take her.

Frederic espies a group of girls, all wards of Major-General Stanley.  Frederic sees their beauty - and Ruth's plainness - and renounces her.  Of the girls, Mabel takes a particular interest in Frederic, and he in her.  The other girls are seized by the pirates and threatened with immediate marriage.  When the Major-General arrives, he can dissuade the pirates only by a ruse:  he tells them that he is an orphan, and so works upon their sympathies that they let him and his wards go free.

The Major-General was greatly troubled by his deception, and feared the return of the pirates.  He summed the police to aid him and Frederic joined their ranks to exterminate the band and defend the woman he loves.

Meanwhile, the Pirate King and Ruth return to speak with Frederic: they have discovered that his indentures were to run until his twenty-first birthday, and - as he was born on February 29 - he has really had as yet only five birthdays.  Obeying the dictates of his strong sense of duty, he immediately rejoins the pirates.  He tells them of the deception that has been practiced upon them, and they swear revenge.

Frederic bids farewell to Mabel and promises to return to her when he is of age, and she in turn swears to be true until then. The pirates return to punish the Major-General and claim all of his wards as brides, but the police arrive and command the pirates to surrender in the name of Queen Victoria, which they do.  Ruth explains, however, that these men who appear to be lawless pirates are really all "noblemen who have gone wrong", and they are pardoned and permitted to marry the Major-General's wards.