Act I takes place on a rocky seashore on the coast of Cornwall. As the curtain rises a group of pirates are drinking and playing cards. Frederic, an apprenticed pirate, sits looking miserable with Ruth, a maid of all work, seated at his feet.

The Pirate King congratulates Frederic on becoming a fully qualified pirate. Frederic replies that he has only done his duty as an apprentice but has always secretly loathed robbing and pillaging and intends leaving the pirates. He admits that his apprenticeship has been due to a mistake made by his nursemaid Ruth, but will not explain any further as it reflects badly upon her. Ruth admits that when she was a nursery maid she misheard her instructions and apprenticed Frederic to a pirate instead of a pilot.

Frederic tells the pirates of his dilemma. Although he likes them individually he must devote himself to their extermination. The pirates complain that they cannot make piracy pay, but Frederic reminds them that they are too soft-hearted with their enemies, especially when their captives happen to be orphans. As orphans themselves they refuse to harm anyone who admits to being an orphan. This weakness has now become common knowledge, and all their victims claim to be orphans in order to escape.

Frederic prepares to leave the pirate band and is persuaded to take Ruth with him. Never having seen another woman, he is unsure as to her beauty although she assures him that her age and appearance should not stand in the way of their marriage.

In the distance they hear the voices of young ladies and conceal themselves. These are the daughters of Major-General Stanley and they picnic on the shore. Frederic, amazed by their beauty, introduces himself, begging one of the young ladies to take pity on him. If one of them will marry him he will completely reform his way of life. Mabel agrees and while the girls gather round him, the pirates enter stealthily and seize the remaining girls. They persuade the girls that they too should do likewise. Major-General Stanley arrives and objects to having pirates for sons-in-law. He tells them he is an orphan and they give him permission to leave, taking his daughters with him.

Act Two

Act 2 takes place in a ruined chapel at night. Major-General Stanley is wracked by guilt at having lied about being an orphan. Mabel and Frederic try to comfort him. Frederic announces that he is to lead an attack by the police against the pirates that very night. The Pirate King and Ruth enter and inform Frederic that he is still an apprentice pirate. As he was born on February 29th in a leap year, he has only served five of his twenty-one years. Frederic agrees to go back to the pirates and tells them that Major-General Stanley lied about being an orphan. Frederic pleads in vain for clemency with the Pirate King, who is resolved to attack the castle. Mabel tries to persuade Frederic not to return to piracy, saying that she will wait for him until 1940, if he is still alive. She tells the Sergeant of Police of Frederic’s heroic sacrifice to “duty”. The police prepare to mount their attack on the pirates, bemoaning their daily task of depriving people of their liberty. In the ensuring struggle it is disclosed that the pirates are really noblemen who have gone wrong. Everyone is forgiven and the ex-pirates win the girls.