The Pirates of Penzance tells the story of a young pirate apprentice named Frederic who has come to the end of his indentured period. As it turns out, Frederic was indentured by mistake. His half-deaf nurse had been instructed to apprentice him to a"pilot" but got it a little wrong. At any rate, Frederic has decided to leave the pirate life forever and, though he loves his comrades dearly, devote his life to the extermination of their kind. However, since he is until the stroke of midnight still one of them, he feels obliged to point out the pirates' weakness--they are too softhearted. Apparently, all a captive must do is plead to being an orphan and he is immediately released!

Once upon the shore, Frederic, who has never seen any woman other than his old nurse, stumbles upon a bevy of beautiful maidens. He immediately falls in love with the most beautiful of the bunch, Mabel, who graciously offers to reclaim the "Poor Wandering One." What Frederic has forgotten, however, is that there are pirates about! Suddenly his old comrades are upon them! The lonely pirates are delighted by the beauty of their captives and recognize the situation as a"first rate opportunity of getting married with impunity." But the girls' father, Major-General Stanley, arrives just in the nick of time and claims untruthfully to be an orphan, thus winning a brief reprieve for his daughters. The General is terrified that the pirates will uncover his lie, but Frederic eases his fears, promising to apprehend the band of pirates and put an end to their plundering. However, when Frederic learns that, due to a technicality, he is still indentured to the pirates, the complications abound.