A group of pirates celebrate the coming of age of their apprentice, Frederic. But Frederic has only been a pirate because of his sense of duty. He plans to go to the police and have them arrested. The arrival of a chorus of beautiful 17-year old girls on a beach outing with their guardian, Major-General Stanley, leads to one complication after another. The opera ends with a hilarious battle with the police, and the discovery that the pirates are, after all, "Noblemen who have gone wrong."