Show dates: April 2001
On the coast of Cornwall, during Queen Victoria's reign, Frederic celebrates the completion of his twenty-first year and the end of his apprenticeship to a gentlemanly band of pirates. The pirates' maid of all work, Ruth, appears and reveals that, as Frederic's nursemaid long ago, she made a mistake "through being hard of hearing": Mishearing Frederic's father's instructions, she apprenticed him to a pirate, instead of to a ship's pilot.
Frederic has never seen any woman other than Ruth, and he believes her to be beautiful. The pirates know better and suggest that Frederic take Ruth with him when he returns to civilisation. Frederic announces that, although it pains him, so strong is his sense of duty that, once free from his apprenticeship, he will be forced to devote himself to the pirates' extermination. He also points out that they are not successful pirates: since they are all orphans, they allow their prey to go free if they too are orphans. Frederic notes that word of this has got about, so captured ships' companies routinely claim to be orphans. Frederic invites the pirates to give up piracy and go with him, so that he need not destroy them, but the Pirate King says that, contrasted with respectability, piracy is comparatively honest. The pirates depart, leaving Frederic and Ruth. Frederic sees a group of beautiful young girls approaching the pirate lair, and realises that Ruth misled him about her appearance. Sending Ruth away, Frederic hides before the girls arrive.
The girls burst exuberantly upon the secluded spot. Frederic reveals himself, startling them. He appeals to them to help him reform. The girls are fascinated by him, but all reject him, except one: Mabel, responds to his plea, chiding her sisters for their lack of charity. She offers Frederic her pity, and the two quickly fall in love. The other girls discuss whether to eavesdrop or to leave the new couple alone, deciding to "talk about the weather," although they steal glances at the affectionate couple.
Frederic warns the young ladies that his old associates will soon return, but before they can flee, the pirates arrive and capture the girls, intending to marry them. Mabel warns the pirates that the girls' father is a Major-General, who soon arrives and introduces himself. He appeals to the pirates not to take his daughters, leaving him to face his old age alone. Having heard of the famous Pirates of Penzance, he pretends that he is an orphan to elicit their sympathy. The soft-hearted pirates release the girls, making Major-General Stanley and his daughters honorary members of their band.
The Major-General sits in a ruined chapel on his estate, surrounded by his daughters. His conscience is tortured by the lie that he told the pirates, and the girls attempt to console him. The Sergeant of Police and his corps arrive to announce their readiness to arrest the pirates. The girls loudly express their admiration of the police for facing likely slaughter at the hands of fierce and merciless foes. The police are unnerved by this but finally leave.
Left alone, Frederic, who is to lead the police, reflects on his opportunity to atone for a life of piracy, at which point he encounters Ruth and the Pirate King. They have realised that Frederic's apprenticeship was worded so as to bind him to them until his twenty-first birthday – and, because that birthday happens to be on the 29th of February (in a leap year), it means that technically only five birthdays have passed, and he will not reach his twenty-first birthday until he is in his eighties. Frederic is convinced by this logic and agrees to rejoin the pirates. He then sees it as his duty to inform the Pirate King of the Major-General's deception. The outraged outlaw declares that the pirates' "revenge will be swift and terrible".
Frederic meets Mabel, and she pleads with him to stay, but he feels bound by his duty to the pirates until his 21st birthday – in 1940. They agree to be faithful to each other until then, though to Mabel "It seems so long"; Frederic departs. Mabel steels herself and tells the police that they must go alone to face the pirates. They muse that an outlaw might be just like any other man, and it is a shame to deprive him of "that liberty which is so dear to all". The police hide on hearing the approach of the pirates, who have stolen onto the estate, intending to avenge themselves for the Major-General's lie.
Just then, Major-General Stanley appears, sleepless with guilt, and the pirates also hide, while the Major-General listens to the soothing breeze. The girls come looking for him. The pirates leap to the attack, and the police rush to the defence; but the police are easily defeated, and the Pirate King urges the captured Major-General to prepare for death. The Sergeant has one stratagem left: he demands that the pirates yield "in Queen Victoria's name"; the pirates, overcome with loyalty to their Queen, do so. Ruth appears and reveals that the pirates are "all noblemen who have gone wrong". The Major-General is impressed by this and all is forgiven. Frederic and Mabel are reunited, and the Major-General is happy to marry his daughters to the noble pirates after all.