Sailors work on the deck of the H.M.S. Pinafore off the coast of Portsmouth. Little Buttercup enters with her wares Ralph enters and reveals that he loves Josephine, the Captain’s daughter. Captain Corcoran enters, and the sailors cheer him. After the sailors leave, he confides in Buttercup that Sir Joseph wishes to marry Josephine, but she does not love him. Josephine enters, bemoaning her fate, and confesses to her father that she loves a common sailor. He warns Corcoran not to patronize his men; all seamen are equal, and the fact that some are captains and some are sailors is a mere accident of birth. He gives Ralph and the other sailors a song to remind them of this fact (“A British tar is a soaring soul”. Emboldened by Sir Joseph’s words, Ralph expresses his love to Josephine; but, knowing that she must marry Sir Joseph, she rejects him. He is about to kill himself out of grief, but Josephine reveals that she loves him after all. They plan to elope, although Dick warns them that such a plan will go awry. At night, the Captain sings to himself as Buttercup watches him; he thinks to himself that he might love her, if not for the difference in their rank. She prophecies that changes will come for him. Sir Joseph expresses his disappointment in Josephine’s behavior; Corcoran suggests that she is merely intimidated by Sir Joseph’s rank. Josephine enters, anticipating her elopement with Ralph. Sir Joseph reassures her that a difference in rank is no impediment to a marriage, and she notes “how eloquently he has plead his rival’s cause.” Dick reveals Ralph and Josephine’s plan to the Captain and they watch as the couple makes ready to leave. Corcoran confronts Ralph and becomes so angry that he swears. This distresses Sir Joseph, who sends Corcoran to his cabin. Ralph and Josephine reveal their love to Sir Joseph, who orders Ralph thrown in the dungeon. Ralph says farewell to Josephine. However, Buttercup steps forward to reveal that Captain Corcoran and Ralph were accidentally switched at birth, and so the Captain is merely a common sailor. Sir Joseph rejects Josephine on account of her low rank, and she and Ralph are free to marry. The Captain promises to marry Buttercup, and Sir Joseph promises to marry Hebe.