The plot of HMS Pinafore in a minute:

Aboard the HMS Pinafore, there is a very clear class divide between the crew and their Captain. So when sailor Ralph Rackstraw falls in love with the Captain’s daughter Josephine, their prospect of getting married doesn’t look so good. In fact, the Captain has already promised Josephine’s hand to Sir Joseph, a high-ranking officer in the Navy.

What do Ralph and Josephine do? Try to elope, of course! But they get caught, causing both the Captain and Sir Joseph to be furious. In comes Little Buttercup with a startling revelation — many years ago, when Ralph and the Captain were babies, she had swapped them, meaning Ralph was actually of high birth and the Captain was not. All is resolved! Now that the tides have turned, Ralph is allowed to marry Josephine and everyone lives happily ever after.

Detailed synopsis:

Act One

The crew are singing and going about their day when a strange woman named Little Buttercup arrives on the HMS Pinafore. Ralph Rackstraw enters and tells everyone he is in love with the Captain’s daughter Josephine. She doesn’t know it yet, but they couldn’t be together anyway because Josephine is of high birth while Ralph is a mere sailor.

The Captain enters and his crew welcomes him. Little Buttercup asks why the Captain looks sad. He confides that Josephine doesn’t seem happy about her hand being promised to Sir Joseph, a high-ranking officer in the Navy.

Alone, Josephine reveals her love for Ralph Rackstraw. When she confronts her father with this information, he is not happy. Josephine promises to bury her feelings and marry Sir Joseph.

Sir Joseph and his family arrive with much fanfare. He sings about his high rank and how he was able to get it by doing next to nothing.

In front of the crew, Sir Joseph puts the Captain on blast and asks him to be more respectful to these hardworking men. According to Sir Joseph, all men are equal and should not be separated by class. Ralph takes this to heart and believes he can pursue Josephine despite her higher status.

Alone, Josephine laments her lack of feelings for Sir Joseph. Ralph, who has been listening, shows himself and declares his love for Josephine. Determined to obey her father, however, Josephine acts affronted and rejects Ralph.

Ralph is heartbroken. He tells his crewmates he cannot live without Josephine and will thus take his own life. Just as he is about to, Josephine rushes in and declares her love for Ralph. They decide to elope and the crew agrees to help them do it.

Act Two

The Captain is by himself, feeling a little blue. Buttercup, who has been watching him, comes in and offers him comfort. The Captain says if circumstances were different and they were of the same class, he and Buttercup could be together.

Sir Joseph is upset because he feels Josephine doesn’t love him. The Captain assures him she does, suggesting she might have been intimidated by Sir Joseph’s rank, which makes even her high birth look low by comparison.

Sir Joseph tells Josephine she shouldn’t feel intimidated by his rank. He even says, “Love is a platform upon which all ranks meet.” Josephine takes this as a sign — she must be brave and marry Ralph.

Unfortunately, the lovers are discovered while they are trying to elope. Sir Joseph has Ralph locked up.

As tensions rise, Little Buttercup enters and says she has a major confession to make. Buttercup reveals that years ago when she worked in the ‘baby farming’ business, she had two baby boys in her care, one of high birth and one of low birth. One day, she swapped the two, meaning they both grew up in households not of their original class. These two boys were, in fact, Ralph Rackstraw and the Captain. Everyone is shocked to learn this truth.

Now that class is no longer a barrier, Ralph is free to marry Josephine, who was easily turned away by Sir Joseph for not being high born. Little Buttercup and the Captain are also now able to profess their love and be together. Everyone is happy and the HMS Pinafore sails off into the sunset!