ACT I

The sailors on the H.M.S. Pinafore are scrubbing the decks and polishing the brass, preparing for the arrival of Sir Joseph Porter, the First Lord of the Admiralty (“We Sail The Ocean Blue”). Sir Joseph is coming today to ask for the Captain's daughter's hand in marriage. It is payday on the Pinafore, and Buttercup, a bumboat gypsy woman, arrives to sell her wares (“I'm Called Little Buttercup”). Buttercup alludes to a dark secret she is hiding. It appears that the entire crew owe money to Dick Deadeye. Dick is a fellow seaman, the ship's contraband supplier, money lender, and mischief maker (“Disagreeable Man”).

We next meet Ralph Rackstraw, pronounced "Raife." He loves the Captain's daughter Josephine, but knows that she is much above his station (“A Maiden Fair To See”). Dick Deadeye firmly believes in England's class system and he tells Ralph that his love for the high-born beauty is doomed, "a foremast hand don't marry no Captain's daughter."

Captain Corcoran appears on deck to inspect his crew (“My Gallant Crew”). The Captain confides in Buttercup that his daughter is showing little interest in the prospect of marriage to Sir Joseph Porter. Josephine appears and tells her father that she loves a humble sailor on board his own ship (“Sorry Her Lot”), but she promises the Captain that the sailor shall never know it.

The Captain prepares his welcome speech for Sir Joseph as the crew get ready for inspection (“Over the Bright Blue Sea/Sir Joseph's Barge Is Seen/Now Give Three Cheers”). Sir Joseph's entourage precedes him.

They are his sister, his cousin, and his aunt (“Gaily Tripping”). Sir Joseph comes on board and explains his remarkable rise in rank from office boy to First Lord ("When I Was A Lad"). Sir Joseph is impressed with Ralph and tells him that a British sailor is any man's equal. This idea prompts Ralph to plan to declare his love for Josephine, and all the crew approve, except for Dick Deadeye. The crew then sing the song that Sir Joseph has written to encourage independence of thought and action in the lower ranks (“A British Tar”).

Ralph confesses to Josephine that he loves her, but she keeps her promise to her father and haughtily rejects Ralph (“Refrain, Audacious Tar”). The crew tries to comfort Ralph, though Dick Deadeye rubs salt in the wound (“I Told You So”). Ralph is inconsolable and opts for suicide. (“Can I Survive This Overbearing”). Just in time, Josephine confesses her love for her able seaman. The couple decide to elope that night with the help of the ship's crew. Only Dick Deadeye opposes the couple's plans. All others hail the loving couple (“Let's Give Three Cheers For The Sailors Bride”).

ACT II

It is the evening and Sir Joseph teaches the Captain to dance a hornpipe while Josephine is torn between her love for Ralph and her sense of duty to her father and her class (“The Hours Creep On A Pace”). The crew gamble their wages away in Dick Deadeye's illegal casino (“The Roulette Song”).

The Captain is at his wits' end with his crew acting strangely, and Sir Joseph feeling rejected by the Captain's unhappy daughter (“Fair Moon To Thee I Sing”). In the shadows, Little Buttercup reveals her tenderness for the Captain, but she too is trapped in the tyranny of class and cannot declare her true feelings. Buttercup warns the Captain not to take things at face value (“Things Are Seldom What They Seem”).

Ralph and Josephine face the fact that they either accept the world's sorrow and restrictions or they follow their hearts (“In Sailing O'er Life's Ocean Wide/The World Is But A Broken Toy”). Sir Joseph is ready to give up pursuing Josephine, but the Captain and Dick Deadeye convince him that she may feel intimidated by Sir Joseph's exalted position. If Sir Joseph would tell her of his belief that love levels all rank, she might have a change of mind ("Nothing Ventured Nothing Gained"). Sir Joseph agrees and approaches Josephine, relaying the idea that love levels all rank. This convinces Josephine to follow her heart and marry her true love, Ralph (“Never Mind The Why Or Wherefore”).

Dick Deadeye tells the Captain of Josephine and Ralph's plans (“Kind Captain, I've Important Information”). The furious Captain hides in order to catch the elopers (“Carefully On Tiptoe Stealing”). Captain Corcoran denounces his daughter and her fiancé, but they protest that according to Sir Joseph's philosophy, a British sailor is the equal of anyone in the world ("He Is An Englishman”).

Affronted by events, the Captain swears, only to be sent to his cabin by Sir Joseph. When Sir Joseph learns of Ralph's love for Josephine, he sends Ralph to the ship's prison. At this point, Little Buttercup reveals her terrible secret (“A Many Years Ago”). When she was a nurse, she carelessly mixed up two babies, one of low birth, the other a patrician. The two babies were Ralph and Captain Corcoran. As a result, Ralph is really the high-born Captain, and the Captain is none other than the common Ralph.

In light of these circumstances, Sir Joseph finds it impossible to marry the low-born Josephine, so he hands Josephine over to Ralph. That also means that the former high born Captain is a common sailor and able to wed Little Buttercup.

Sir Joseph laments that he will have to spend the rest of his days alone. But Dick assures Sir Joseph that what he needs is a Secretary of the Navy to look out for Sir Joseph and his sister, his cousin, and his aunt. Dick accepts the position immediately (“Oh Joy, Oh Rapture Unforeseen!").