Lying at anchor in Portsmouth harbor, the Pinafore is the scene of hectic activity. Sir Joseph Porter, K.C.B., First Lord of the Admiralty, has announced his intention to visit the ship. The sailors swab the decks and are inspected by the captain, who is as content with them as they are with him. One member of the crew, however, is far from happy. Ralph, the lowly foremast hand, is sunk in gloom and despair. He loves Josephine, the captain’s daughter, but because of his low rank she repulses his advances and rejects his love.

Before Sir Joseph’s arrival, Little Buttercup comes on board, plying her trade as a seller of ribbons and laces, scissors and knives, treacle and toffee. In a conversation with the captain she hints that appearances are often deceiving. The captain notices that Little Buttercup has physical charms not displeasing to him. Sir Joseph’s barge approaches, and the First Lord is soon on board, accompanied by his sisters, his cousins, and his aunts. After inspecting the crew, he gives them instructions for success. His own formula is simple enough: He polished door handles, stuck close to his desk, and never went to sea. Sir Joseph then proceeds to the purpose of his visit: He comes to ask Josephine to marry him.

Josephine has no intention of marrying Sir Joseph, whom she dislikes. Not able to give an outright refusal, she informs him that marriage with such a high-ranking officer is impossible because she is only a captain’s daughter. Sir Joseph admires her modesty but brushes aside the objection. Rank, he assures her, is absolutely no barrier, for love levels all rank. Josephine hastens to agree with him, and everyone immediately assumes that a marriage will soon take place.

Giving up all hope of winning Josephine, Ralph puts a pistol to his head and prepares to pull the trigger. At that moment Josephine rushes in, tells him not to destroy himself, and proclaims her undying love for him. At this turn of events there is general rejoicing among Ralph’s messmates, with the exception of an unsavory character by the name of Dick Dead-eye.

The couple lays plans to steal ashore the next evening to be married. Once the ceremony is performed, they reason, nobody can do anything about it. Dick Dead-eye goes to the captain and warns him of the plan. Accordingly, just as the lovers and their accomplices are quietly tiptoeing away, the captain enters, enraged at Ralph’s presumption and at the low company in which he finds his daughter. Ralph is thrown into the brig.

Attracted by the captain’s swearing, Sir Joseph rushes up in time to hear what happened. The sisters, the cousins, and the aunts are horribly shocked. Sir Joseph is equally shocked, so shocked that he administers a very severe rebuke to the captain. In the midst of the argument, Little Buttercup appears. To the astonishment of everyone, she announces that many years ago she was a baby-sitter. Two infants were put into her care, one of lowly birth, the other of high position. She was very fond of one of them, so she changed them around. The captain is really of low birth, and Ralph is the patrician.

This astounding announcement results in an odd situation that is quickly and amicably arranged. The captain changes places with Ralph, who becomes captain instead. Sir Joseph announces that he cannot marry Josephine since she is the daughter of a common sailor. Accordingly, Josephine marries Ralph, the captain marries Little Buttercup, and Sir Joseph marries a well-born cousin.