H.M.S. Pinafore, or The Lass that Loved a Sailor is a comic opera in two acts, with music by Arthur Sullivan and libretto by W. S. Gilbert. It is one of the Savoy Operas, and the first big hit by Gilbert and Sullivan. It opened at the Opera Comique in London on May 25 1878 for a run of 571 performances, which was the second longest run of any musical theatre piece up to that time (after the operetta Les cloches de Corneville). H.M.S. Pinafore was Gilbert and Sullivan's fourth operatic collaboration. Drawing on several of his earlier "Bab Ballad" poems, Gilbert imbued H.M.S. Pinafore with mirth and silliness to spare. The opera's gentle satire reprises and builds on a theme introduced in The Sorcerer – love between members of different social classes. The opera also pokes good-natured fun at the Royal Navy and, in themes to be repeated in the later operas, parliamentary politics and the rise of unqualified people to positions of authority. The title of the work itself is humorous, as it juxtaposes the name of a little girl's garment, pinafore, with the symbol of a naval warship. The plot revolves around a naval captain's daughter who is in love with a lower-class foremast hand (a common sailor, well below officer rank), even though her father intends her to marry the First Lord of the Admiralty, the cabinet minister in charge of Britain's Royal Navy. As with most of the Gilbert and Sullivan operas, a surprise twist changes matters dramatically near the end of the story.