The lowly sailor, Ralph Rackstraw, has fallen in love with Josephine, daughter of the repressed, but ever polite, Captain Corcoran whose social climbing ambitions have caused him to promise Josephine in marriage to the insufferable Sir Joseph Porter, K.C.B, First Lord of the Admiralty. While the crew and Sir Joseph's groupies, referred to as "his sisters and his cousins and his aunts" dance their way across the deck, Josephine promises not to follow her heart in returning Ralph's affection and the Captain reveals his own attraction to the lowly peddler woman, Little Buttercup, who hints that "things are seldom what they seem." Sir Joseph senses Josephine's coolness to his advances, but is convinced by the Captain to make another effort towards her. In a deliberately absurd twist of fate, Little Buttercup reveals that Ralph and the Captain were switched at birth, thereby allowing both Corcorans to marry the objects of their affections, within their own social classes. Sir Joseph is resigned to marrying his most devoted groupie, Cousin Hebe.