Lights reveal a backdrop layered much like a pop-up book. A small cut-out of a pirate ship passes between the cut-out waves of the drop, firing a cannon to mark its entrance. The pirates themselves appear, celebrating because their young apprentice, Frederic, attains adulthood today, and is now a full-fledged member of their band. But Frederic announces that he will leave the pirates immediately. He had done his best for the pirates until now only out of his deep devotion to his duty under his apprenticeship which, after all, had been a mistake.

The pirates are deeply wounded, and Ruth, Frederic's nanny, explains. She had been directed by Frederic's father to apprentice Frederic to a pilot, but misheard, and bound him to a pirate instead.

Frederic forgives Ruth her mistake, and addresses the pirates: "Individually, I love you all with affection unspeakable, but, collectively, I look upon you with a disgust that amounts to absolute detestation." He tearfully vows to exterminate them all. But, since the time is only half-past eleven and he is a pirate until twelve, he explains why they don't make very good pirates: their reputation for nobleness has gotten about, so that every person captured claims to be an orphan and invokes the pirates' mercy.

Ruth is worried about her own fate, and proposes herself in marriage to Frederic. Frederic accepts, but with some reservation. Ruth is the only woman he knows; what if he meets another and finds out that Ruth is plain? The pirates, only too glad to let her go, assure him that "there are the remains of a fine woman about Ruth." The Pirate King bids Ruth and Frederic farewell, joyously returning to his pirate's life .

Left alone, Frederic seeks reassurances from Ruth about their upcoming alliance, which she gives as best she can without outright lying. Their parley is interrupted by singing voices, and Frederic spies a group of beautiful young women. Looking upon them, he realizes he has been tricked by Ruth, and dismisses her. Frederic, ashamed of his piratical appearance, hides from the approaching young women.

Major-General Stanley's daughters enter, seeking a secluded spot for a picnic. Seeing them about to take off their shoes and play in the water (how indecent!), Frederic reveals himself to them, warning them that their picnic spot is a pirates' lair. The daughters are frightened of him as a pirate, but attracted to his youthful good looks. He proposes marriage to all of them at once, but they reject him.

Suddenly, Mabel appears. She is another of the Major-General's daughters, and she is astonishingly beautiful. She chides her sisters for their lack of pity, and consoles Frederic. As Frederic and Mabel make each other's acquaintance, the other daughters pretend to give them some time alone while they chat about the weather.

Frederic hears the pirates approaching, and urges the daughters to flee before them. But they stand so long singing about their need to escape that they fail to actually do so. The pirates capture the girls, although their notion of "rape" is to carry the girls off to a minister and marry them on the spot. Mabel stops them, warning them that their father is an illustrious personage: a Major-General! And he appears, boasting of his knowledge and abilities.

Once the Major-General has been apprised of the situation, he objects to having pirates as sons-in-law. The Pirate King responds, "We object to Major-Generals as fathers-in-law. But we waive that point." The Major-General begs them not to rob him of his daughters, "the sole, remaining propos of [his] old age," as he is (of course) an orphan boy. The pirates let him go, and everybody celebrates, except poor abandoned Ruth.

The Major-General's daughters try to comfort him. He is disconsolate because he has dishonored his ancestors by escaping the pirates through a lie. Frederic reminds him that he has only recently bought his lands and title, and therefore cannot do too much damage to his family legacy. The Major-General explains that there are indeed somebody's ancestors buried on his land, and since he bought the land and all its contents, they are therefore his ancestors.

The Major-General says he will be much more comfortable when Frederic has exterminated the pirates. Frederic summons his army of policemen, who are less than eager to go to battle. Mabel and the other daughters urge them on, and the policemen vow to go and do their duty. But they keep vowing for a long time before they are finally persuaded to go.

Frederic's relief at finally atoning for his years of piracy is interrupted by none other than Ruth and the Pirate King. They know of Frederic's love of paradoxes, and propose a most ingenious one to him. It seems Frederic was born on Leap Day, February 29, which only comes once every four years. Frederick is twenty-one years old, therefore, but has had only five birthdays. Unfortunately, the terms of his pirate apprenticeship are until his twenty-first birthday. By the terms of his indenture, he will remain a pirate until he is eighty-four years old! Frederic, bound by his sense of duty, agrees to go with them, bemoaning his fate and his consequential loss of Mabel. Further constricted to duty, Frederic informs the Pirate King of the Major-General's deception. The Pirate King vows revenge.

Frederic is left alone with Mabel, and he informs her of the sad situation. She begs him to ignore his duty, but he cannot. They vow to remain true until Frederic is free to marry her, in 1940. Frederic leaves to join the pirates, and Mabel mourns. But she is determined to go on and urges the policemen on to fight the pirates even though their commander has switched sides. The policemen don't like their job - after all, criminals are people too - but they set off to do it.

The pirates approach to get their revenge, and the policemen bravely hide. The pirates plot their burglary. Frederic quiets them as the Major-General approaches, kept from his sleep by a guilty conscience. Just as his daughters come out to comfort him, the pirates ambush them, and are in turn ambushed by the police.

A fight ensues. The police duel with billy clubs, the daughters with parasols. The pirates win the battle, but the sergeant of the police calls on them to yield in Queen Victoria's name. The pirates surrender immediately, because "with all their faults, they love their queen."

Before the pirates can be led away, Ruth steps forward and reveals them as prodigal sons of titled families. The pirates are instantly forgiven, as "peers will be peers." Frederic is reunited with Mabel, Ruth is swept up by the sergeant, the pirates and daughters pair off, and there is a happy ending for all.