High Spirits seem to be the order of the day aboard Her Majesty's Ship Pinafore, anticipating the visit of Sir Joseph Porter, First Lord of the Admiralty. The crew is busy preparing the ship, stopping only to make purchases from Little Buttercup, a peddler, and to exchange compliments with their well-mannered Captain.

The only sad note is struck by Ralph Rackstraw, one of the sailors, who is hopelessly in love with Josephine, the Captain's daughter. Not only is her social position far above his, but her hand has been promised by her father to Sir Joseph.

When Sir Joseph arrives with his female entourage, professing some decidedly eccentric ideas about naval discipline, Ralph is inspired to press his suit with the lady. At first she refuses him, but when he threatens suicide, she admits her love for him and agrees to elope.