The Pirates of Penzance; or, The Slave of Duty is a comic opera in two acts, with music by Arthur Sullivan and libretto by W. S. Gilbert. The story concerns Frederic, who, having completed his 21st year, is released from his apprenticeship to a band of tender-hearted pirates. He meets Mabel, the daughter of Major-General Stanley, and the two young people fall instantly in love. Frederic finds out, however, that he was born on 29 February, and so, technically, he only has a birthday each leap year. His apprenticeship indentures state that he remains apprenticed to the pirates until his 21st birthday, and so he must serve for another 63 years. Bound by his own sense of duty, Frederic's only solace is that Mabel agrees to wait for him faithfully. (Summary from Wikipedia)