A rocky seashore on the coast of Cornwall
The pirates offer a toast to Frederic who is celebrating his 21st birthday and the end of his apprenticeship. Ruth, a maid who also works with the pirates, explains that when Frederic was younger she was instructed by his father to have the small boy apprenticed to a pilot, but she misheard and instead took Frederic to a pirate.
Now that his obligation to the pirates has ended, duty compels Frederic to devote himself to destroying them. When the men admit their inability to make piracy pay, Frederic explains that they are themselves slaves to their tender hearts, particularly regarding their refusal to take advantage of anyone they believe to be an orphan. Frederic wishes he could bring the pirates back to a respectable life, but the Pirate King rejects that possibility.
Ruth longs for Frederic to take her with him when he leaves. He admits that he is reluctant to marry her, though he is satisfied that, despite her age, she is beautiful. But when he sees a group of lovely young women approaching, he rages at her for deceiving him.
The young women are astonished to encounter Frederic and horrified to hear that he is a pirate. He assures them that he only wants to love one of them. They all reject him except Mabel.
The pirates surprise the girls and anticipate soon becoming their husbands. Their father, the Major General, enters and objects to the girls being married against their wills to pirates. He lies his way out of the situation by claiming to be an orphan. The girls are released from the pirates’ clutches, as Mabel and Frederic – ignoring the pleas of Ruth – look forward to their marriage.
On Major-General Stanley’s estate: a ruined chapel by moonlight
The Major-General believes his lie about being an orphan has shamed the family name and he fears the consequences. Frederic will soon march against the pirates accompanied by the police.
Ruth and the Pirate King enter. They explain that Frederic was to be apprenticed until his twenty first birthday but having been born in a leap year on 29 February, he is officially only five. The Pirate King insists that Frederic respect his own sense of duty, whereupon the obedient young man informs him that the Major General lied about being an orphan. The Pirate King and Ruth swear vengeance.
Frederic informs Mabel that he will not reach his twenty-first birthday until 1940. Convinced that he can ignore the pirates’ claim, she begs him to remain with her and swears to be faithful, even though he is a slave of duty and feels compelled to re-join the pirates.
Afflicted by insomnia, the Major General enters. Frederic and the pirates seize the Major General and overcome the police. When the Sergeant implores the pirates to yield in the Queen’s name, Ruth reveals that they are in fact peers of the realm. With this revelation, they are pardoned by the Major-General, who rewards them with the girls’ hands in marriage.