Frederic has just completed his pirate apprenticeship and is being hailed as a fully-fledged member of the gang. Unfortunately, Frederic must refuse—his apprenticeship had been a mistake to begin with (his hard-of-hearing nurse had taken him to a “pirate” rather than a “pilot”), and Frederic only stayed because he was duty-bound to do so. Now that he is free of his indentures, the young man feels morally obligated to hunt down and exterminate his former comrades. The pirates are heartbroken, but understand Frederic’s predicament, and leave him on the beach with Ruth, his childhood nurse. The middle-aged Ruth offers herself in marriage to Frederic; however, having never seen another woman’s face he is understandably hesitant. The sound of approaching voices spells doom for Ruth’s plans—it is a bevy of beautiful girls who have come to the rocky cover for a picnic. Frederic’s wonder overcomes his self-consciousness about his attire and he steps forward to ask if any of the maidens would stoop in pity so low as to accept the hand and heart of a pirate. Only one of them, Mabel, is ready to take him for what he is. Of course, they fall in love at once. Their courtship is interrupted by the untimely return of the pirate gang, each member of which seizes a girl and claims her as his bride. Luckily for the girls, their guardian, Major-General Stanley, has caught up with them. Although an open-minded man, the “very model of modern major-general,” the General objects to pirates as sons-in-law. Luckily he has heard of these Penzance pirates before—being orphans themselves, they are notorious for their sympathy towards orphans. He declares that he too is “an orphan boy,” and the tender-hearted pirates surrender the girls and make the Major-General an honorary member of their band.
Weeping in the ancestral tomb of his (newly acquired) estate, Major-General Stanley is wracked by guilt over his lie to the pirates. Frederic is now to lead an expedition against the pirates accompanied by a squad of loyal but rather timid policemen. However, before he can depart on his mission, the Pirate King and Ruth appear with a curious paradox: Frederic was apprenticed to them until his 21st birthday—which is significant because having been born in a Leap Year on February 29th, he has technically had only five birthdays and thus still a Pirate. Ever the slave of duty, Frederic reluctantly agrees to return to the gang to finish his apprenticeship. He feels bound also to reveal that Major-General Stanley is not, in fact, an orphan.
The Pirate King, furious that their tenderness has been abused, vows a terrible vengeance and rushes off with Ruth to prepare the attack. Frederic lingers to say a teary farewell to Mabel, who promises to stay faithful (even though Frederic will not have 21 birthdays until the distant 1940). The police return in time to hide, for a series of roars and crashes herald the pirates’ imminent “sneak attack.” Soon, Major-General Stanley, his daughters, and the policemen are overcome and held at sword-point. The police sergeant calls upon the ruffians to surrender in the name of the Queen. The command acts like magic. Loyally the pirates kneel to their captives, for it transpires that they are all in fact noblemen who have gone wrong. Major-General Stanley bids them rise as peers are entitled their youthful indiscretions and promises that if they return to their duties in the House of Lords, all will be forgiven. Frederic and Mabel are re-united, and the pirates and other daughters pair off as everyone imagines a very large wedding.