Act I - The Quarter Deck of H.M.S. Pinafore, Noon

H.M.S. Pinafore, a British navy man-o-war, has just returned to Portsmouth harbour. As the curtain rises, the sailors are discovered busily cleaning and polishing the ship. Mrs. Cripps, known to all as "Little Buttercup," comes on board to sell all sorts of goodies to the sailors. Despite her cheerful nickname, she hints at a great sorrow which afflicts her. The misanthropic and ugly Dick Deadeye intervenes, but his opinions are roughly rejected by his messmates.

The handsome, but low-born Ralph (pronounced "Raiph") Rackstraw enters and sings of his love for the captain's daughter, whose social position makes his suit hopeless. The others, expecting Dick, express their sympathy.

Captain Corcoran now appears, and exchanges most polite civilities with the crew and sings of his good breeding. However, he is a little sad because although his daughter Josephine is engaged to marry the illustrious First Lord of the Admiralty, Sir Joseph Porter, she is singularly lacking in enthusiasm for the match, despite its immense social advantages. She, herself, arrives and sings a melancholy song of hopeless love. She confesses that she loves a common sailor, but reassures her horrified father that she will never reveal this secret.

Preceded by a crowd of female relatives, including the loving Hebe, Sir Joseph, now makes a ceremonial entrance. He describes how legal training and hard work have led him to his exalted post, despite his never having been a sailor himself. He praises the crew fulsomely, demanding the utmost politeness from Corcoran when giving them orders, and, before retiring to the captain's cabin, presents them with a patriotic song of his own composition.

With the crew's approval (except, of course, for Dick Deadeye), Ralph determines to tell Josephine he loves her. Then they all proceed to sing Sir Joseph's rousing song. Josephine comes back, nauseated by Sir Joseph's attentions to her. Ralph declares his love, but although she loves him, she proudly spurns his pleas, and runs off. He prepares to kill himself, with the crew looking sadly on, but Josephine, watching from the side, rushes on to prevent him and to confess that she returns his love. Despite Dick Deadeye's dire warnings, they decide to elope that very night, and the act ends with general rejoicing.

Act II - The Same, Late That Night

Captain Corcoran is discovered, singing perplexedly to the moon of his problems. He tells Buttercup that were there not such a gap in their social status, he would love her, and she then hints, in a mysterious song, that things are not as they appear to be.

Sir Joseph is disappointed with Josephine's coldness to him, but Corcoran suggests that she is abashed by his august position. After she enters and sings of her love for Ralph, Sir Joseph assures her that difference in station is no bar to love. She welcomes this as vindication of her love, and joins in a spirited trio on how love can level all ranks.

When the captain is alone, Dick Deadeye tells him of the elopement plan. Corcoran takes a cat-o-nine-tails and hides, while the assembled company creeps on, ready to assist the lovers. He confronts them, and in his anger uses vulgar language which is overheard by Sir Joseph. Very shocked he orders Corcoran to his cabin, but on learning the reason, orders Ralph arrested and thrown into the ship's dungeon.

Buttercup now reveals that, as babies, the captain and Ralph were switched around. The captain should thus really be a common sailor, and Ralph should be the captain. The two appear, with their roles reversed. Sir Joseph cannot now marry a humble tar's daughter, but Ralph can. As a simple crew member, Corcoran can now marry Buttercup, and, needing a wife, Sir Joseph accepts the adoring Hebe. Dancing, jollity, curtain!