Act I - A Rocky Sea Shore on the Coast of Cornwall

As the curtain rises a band of pirates is celebrating the coming of age of Frederic, their apprentice from the age of eight. Today he is twenty-one and out of his indentures. He thanks them for their congratulations, but says that he must leave them forever. Ruth, the pirate maid, explains that by mistake she apprenticed Frederic to them instead of to a pilot. He felt duty bound to serve them but now must work for their destruction unless they abandon their calling. Their king points out that his life is comparatively honest when compared to the cheating world, and the pirates depart.

Ruth begs Frederic to take her with him. She has led him to believe that she is beautiful - he has never seen another woman - but just then a bevy of young women is seen approaching and Frederic realises that Ruth has deceived him. Furiously he rejects her and she leaves in despair. Not wishing to alarm the maidens, Frederic hides. The girls, all daughters of (the amazingly fecund) General Stanley, sing gaily and decide to go paddling. Frederic feels he must reveal his presence, tells them of his past and begs that one of them will offer him her love. The youngest, Mabel, agrees. While the pair sing together of their happiness, the other girls discreetly talk about the weather. Suddenly the pirates return and seize the girls - the will make wonderful brides. Enter General Stanley. After explaining how "modern" he is, Stanley appeals to the pirates' better nature. He tells them he's an orphan (a whopping fib) and, orphans themselves, the pirates tearfully release the girls. The ensemble sings a paean of praise to "Poetry." Ruth makes one more vain appeal to Frederic and the curtain falls on a stirring tableau.

Act II - A Ruined Chapel by Moonlight

General Stanley, comforted by his daughters, is conscience-stricken at having deceived the pirates - but not enough to go to them and confess. Frederic declares that very evening he will attack the pirate lair, aided, naturally enough, by those champions of justice, the British police. They now enter, led by their sergeant, but feel the need to sing valiantly to keep up their courage, especially when the girls exhort them to die gloriously in bloody battle. Most reluctantly they eventually depart to prepare for the coming conflict.

Frederic is left alone, but is surprised by Ruth and the Pirate King. They have discovered a curious paradox - he was born on a leap year on the 29th of February, and since he was apprenticed till his 21st birthday, he is still bound to serve them. Appalled though he is, Frederic agrees to follow them, and reveals General Stanley's deception. After a great revenge trio, he is left alone to bid farewell to Mabel. He tells her the terrible truth and they sing a passionate duet before he rushes away.

Mabel summons the policemen and tells them they will have to fight the pirates on their own. The sergeant then sings of the doleful life of the constabulary. Suddenly the pirates are heard approaching. Despite their claims to a "cat-like-tread" they indulge in a vigorous chorus and the policemen wisely hide. General Stanley enters, sleepless with the pangs of conscience, and sings a doleful song. The pirates seize him, and the constables try to come to his rescue but are swiftly defeated. An appeal to their patriotism, however, works wonders, and when it is revealed that they are all actually peers of the realm, there is general reconciliation and pairing off. The curtain descends on traditional festivity.