The play opens with the crew of the H.M.S. Pinafore polishing brasswork and splicing lines as Little Buttercup arrives with her basket of wares and singing “Poor Little Buttercup.” Dick Deadeye appears, and the crew agrees that he is easily as unpopular as he claims to be. Able seaman Ralph Rakestraw enters next and reveals, through the song “A Maiden Fair to See,” that he is in love with a girl who is, alas, far above his social station. In fact, he is love with none other than Josephine, the captain’s daughter.
Enter Captain Corcoran, who is hardly ever sick at sea. He relates that his daughter is being sought in marriage by Sir Joseph Porter, first lord of the admiralty, but she isn’t taking kindly to it. In fact, his daughter Josephine enters next, with a small basket of flowers. Sadly, she tells her father that she has great respect for Sir Joseph but her heart is given to another, a humble sailor on her father’s own ship. Sadly, she vows to carry her love to the tomb, never revealing it to the one she loves.
Enter Sir Joseph with an entourage of female relatives. Sir Joseph sings “I Am the Ruler of The Queen’s Navee,” revealing that he has absolutely no naval experience. In the speech that follows, he posits a belief that each noble English seaman is only lower than the captain in social rank due to an “accident of birth” and is any man’s equal, including his own. Sir Joseph compliments Ralph Rakestraw as an exemplary seaman. Ralph takes courage from Sir Joseph’s equality speech, and all his mates, except Dick Deadeye, agree that he should make his feelings for Josephine known.
Josephine enters, lamenting Sir Joseph’s advances and in an aside she reveals to the audience that Ralph is the object of her love. In eloquent speech, Ralph declares his love for her. She spurns his advances, calling his attention to the difference in their social stations. Ralph decides that his life no longer has meaning and decides to shoot himself. Josephine reappears, crying “Stay your hand, I love you!”
Act Two finds the captain singing ”Fair Moon, to Thee I Sing” as Buttercup gazes longingly at him. He would love her, the captain declares, were it not for the difference in social stations. Buttercup tells the captain there is a change in store for him as they sing “Things Are Seldom What They Seem”.
Sir Joseph comes to tell that the captain that his daughter just won’t do. The captain urges Sir Joseph to tell Josephine that the difference in their social ranking is not important if two people love each other.
Josephine laments the quandary she finds herself in. Whom should she choose, Sir Joseph with his wealth and social status, or poor Ralph Rakestraw, whom she loves dearly? Sir Joseph, unwittingly, helps her to make up her mind with his assurance that social rank is unimportant.
Dick Deadeye arrives and reveals that Ralph and Josephine are planning to elope. The captain, Sir Joseph, and Dick intercept the young couple, vowing to put Ralph to the lash. The crew sings “He Is an Englishman,” and the captain breaks his vow never to swear. Sir Joseph defends Ralph as anyone’s equal until Josephine appears and rushes to Ralph’s arms, whereupon he demands that Ralph be taken to the dungeon. Buttercup then reveals her long-kept secret. Long ago, she had nursed two infants, one of lowly birth, the other a patrician. She mixed the infants up, switching one for the other. The captain was the common-born child, and Ralph the child of privilege!
Ralph, then, is the one of superior social rank, and can choose to marry Josephine. The captain, being common-born, has no social standing over Buttercup and can marry her. Sir Joseph decides that Hebe, his own cousin, would make a suitable wife and three weddings ensue.