Act One

The setting is Oxford University in the year 1892, where a group of college seniors are bidding farewell to the years gone by ("Salad Days"). In his dorm room, the graduating Jack Chesney is talking excitedly with his butler, Brassett, making sure that luncheon will be ready for later that afternoon. He is so excited, because his roommate, Charley Wykeham, has gone to the train station to meet his aunt, Donna Lucia D'Alvadorez, who is coming in from Brazil for a visit. Having Charley's aunt there as chaperone will make it quite easy for their two girl friends, Kitty Verdun and Amy Spettigue, to come for a visit.

Charley does returns – but without his aunt. Apparently, she was never on the train. After reading her letter more carefully, Charley realizes that he made a mistake – she will be coming on a later one. Not having Charley's aunt there will make it impossible for the girls to stay – no decent girl would remain with two young men, unchaperoned. The girls arrive early and just as expected, decide that they must leave – not seeing Charley's Aunt Donna Lucia ("Better Get out of Here"). Actually, they are more worried about being caught by their guardian, Mr. Spettigue (Amy's uncle), who keeps a tight reign on the girls. The girls do leave.

After the girls' departure, the boys are a bit depressed; nevertheless, they know that the girls will return when Charley's Aunt is there. In the meantime, Charley tries on his costume for the upcoming student production in which he is appearing. While Charley is doing this, Jack's father, Sir Francis Chesney, makes a surprise visit and informs his son that they are actually penniless. Jack suggests that a wealthy marriage could possibly alleviate their problems and suggests that Charley's rich widow Aunt Donna Lucia might just be the woman for his father to wed. All that Sir Francis needs to do is charm her when she comes for lunch later that day. The plan seems to be perfect.

After Jack's father leaves, Charley re-enters in costume, rehearsing his lines for the school play. It appears that he will be playing a rather mature older woman – much like his Aunt Donna Lucia. Just then, he is informed by telegram that his aunt will not be arriving for lunch. Instead, she will surprise him at a later date. Jack is upset most of all, because this will be his last chance to propose to Kitty – tomorrow, she leaves for Scotland. The young ladies unexpectedly appear again, assuming that a chaperone is there, and in a fit of desperation, Jack suggests that Charley pose as his aunt – anyway, he's wearing the costume right now. Charley does pose as his aunt and fools the girls. They cover for Charley, saying that he's not feeling well and has retired to his room.

The girls' guardian, Mr. Spettigue, comes looking for the girls, and everyone rushes out to hide from him – everyone except Charley, who stays to tell Mr. Spettigue that she is "the only young lady present." She finally hits him with her fan and says, "Be gone my good man and don't bother me any longer." Utterly baffled and confused, Mr. Spettigue leaves, and the girls and Jack return. Sir Francis then enters the scene and escorts Charley to lunch, all ready to woo a rich woman into marriage. Just then, Mr. Spettigue re-enters and sees the girls there, just as he suspected. He is quite angry until he learns that this "woman/chaperone" who dismissed him only a few moments ago is actually the wealthy Donna Lucia D'Alvadorez from Brazil – "where the nuts come from." He, too, is smitten by this rich, available widow and is ready to make his move. Together, Sir Francis, Charley and Mr. Spettigue go off to lunch. It's Charley's job to keep them occupied. Jack, Kitty and Amy all go off to watch the parade and look for Charley.

The scene shifts to a street where the students' band is marching. Everyone is excited by what they hear and see ("The New Ashmolean Marching Society and Students' Conservatory Band").

In a garden, Charley is returning from having just entertained both men. Charley is ready to change back into his regular clothes and go find Amy, who must be wondering where he is. Jack is still frantic and begs Charley to continue for a short while longer so that he can talk with Kitty. Charley doesn't care – he has had just about all that he can take and goes off to change and find Amy. Kitty comes looking for Donna Lucia, but instead finds herself alone with Jack. It is there that the two are finally able to express their undying love and devotion for each other ("My Darling, My Darling").

Jack's father finds Kitty and Jack together and asks to speak with his son. It appears as though he is going to take his son's advice and marry Donna Lucia. Jack tells him that this is impossible; however, Sir Francis is now more determined than ever. Charley then appears as himself and finally gets to talk with Amy. He apologizes for his absence and rushes to talk to Amy about their future together before her uncle returns and whisks her away. While Charley only wishes to propose marriage and speak of a future together, Amy only wants to speak of the future – a time of progress with wireless telegraphy, horseless carriages and stereopticons that move ("Make a Miracle").

Mr. Spettigue enters to find Amy with Charley and tells her to collect her things. They are leaving for good! He has been waiting for far too long for Donna Lucia and is now ready to take Amy home. In a desperate moment, Charley tells Mr. Spettigue that his aunt really is in love with him. In fact, she's up in Charley's room right now, resting from the emotional shock of meeting him. He knows that she'll be back soon. It's just that she has some business to attend to – like writing letters to her banks. The greedy Mr. Spettigue agrees to wait and ponders his possible future – and fortune – with Lucia ("Serenade with Asides").

Charley returns as Donna Lucia, and chases Mr. Spettigue around the garden. In the midst of all of this, Jack informs Charley that his father is going to propose to him. In fact, Sir Francis enters and asks Charley to marry him. Charley turns him down but runs off again as he hears a playful Mr. Spettigue approaching.

In the midst of all of this, a woman (Charley's real aunt) enters and approaches Sir Francis about finding Charley. As the two look at each other, they realize that they know one another from more than twenty years ago. Sir Francis doesn't know that she is actually Charley's rich Aunt Donna Lucia; in fact, he points out to her that Charley's aunt is right there with them – running around, being chased by Mr. Spettigue. She is quite intrigued by it all and poses as Mrs. Beverly Smythe to survey the situation better. The two then look at each other once more and recall that time long ago when they first met ("Lovelier Than Ever").

Jack and Charley are going over things, and Kitty and Amy enter to inform Jack that the only way that they'll get to marry the girls is to get Mr. Spettigue's consent in writing. The girls also confide in Charley that they are actually really in love with Charley and Jack, but if they don't get consent to the marriages in writing from Mr. Spettigue, they will be financially cut off for good. Charley agrees to get the written consent from Mr. Spettigue and goes off to do so. Kitty and Jack leave Amy alone to think about Charley and all that is happening. Just where has Charley been? She's crazy about him but can't help thinking that he's off with another girl – possibly the girl who she saw in a picture sitting on his piano ("The Woman in His Room").

Brassett enters and sets tea. Charley returns, still dressed as his aunt, and tells Amy that Charley has sent a message to her – he loves her very much. Sir Francis and the real Donna Lucia (still posing as Beverly Smythe) enter and unexpectedly join them for tea. During a madcap tea party, Donna Lucia presses Charley for information about his/her life in Brazil. Charley tells her about the place where Donna Lucia lived – Pernambuco. A song and ballet sequence (seen through Charley's eyes), revealing just how Donna Lucia and her rich husband met and lived together, ends the first act. ("Pernambuco")

Act Two

The act opens with the Oxford senior graduates posing for their class picture – complete with caps and gowns. They are all there except for Charley. He promised that he'd be there ("Where's Charley?"). Fortunately, he does make it there – just as the picture is being taken.

Walking along a street, Charley tries to explain all of his disappearances to Amy, who tries her best to believe him. She also tells him that her uncle is ready to sign a wedding consent form, but Donna Lucia must meet him to get the document. Charley assures her that "she'll" be there but also tells her that he can't be there at the same time. She doesn't quite know what is going on, but she trusts him and is excited about marrying him. She leaves Charley alone, and there on that street, he thinks about Amy – the most wonderful girl in the whole world ("Once in Love with Amy").

Later that day, the girls are in the dressing room preparing themselves for the evening's big dance. While dressing and primping, they also go over all of the most recent events ("The Gossips"). Kitty drags a reluctant Charley – once again dressed as Donna Lucia – to the dressing room. The real Donna Lucia enters and has a good bit of fun making Charley feel uncomfortable while all the ladies ready themselves for the evening's fun. The ladies all eventually leave, and Charley is left alone to talk with Mr. Spettigue, who comes to play with his new love. Charley fights off Mr. Spettigue's forced advances and finally demands that he be given the letter of consent that has been promised all along. If Mr. Spettigue goes and gets the letter right now, Charley promises that they can announce their own engagement at the ball. Mr. Spettigue dashes off excitedly to get the letter and meet his love at the ball.

Strolling down a garden path, Donna Lucia asks Sir Francis if it isn't strange that they have never seen Charley and his aunt together at the same time. Sir Francis has no interest in any of that. All he cares about is dancing the night away with his long-lost love. The same is true of Jack and Kitty. In fact, everyone is dancing the night away in love ("Red Rose Cotillion"). Charley even makes an appearance as himself to keep poor little Amy happy and content.

Mr. Spettigue finally appears and announces the engagement of Kitty and Amy to Jack and Charley. He gives Charley (again dressed as his aunt) the letter of consent. Charley sneaks off and reappears as himself once more. Mr. Spettigue wants to see Charley's aunt one more time, so Charley goes to get her. Mr. Spettigue asks to see Charley, the nephew, once again, and a rather "tired" Charley dashes off to get him. While rushing, Jack accidentally steps on the hem of Charley's skirt, and all is "revealed." Charley admits to Amy and everyone else that he did it all because of love. Mr. Spettigue is angry and doesn't care that they are in love. Charley tells him that they have the letter of consent, which is all they need. Mr. Spettigue fights them by telling them that he will dispute this. The real Donna Lucia finally steps forward and says that, indeed, the letter of consent was addressed to her and is a legal document – and she puts her blessing on the wedding. There is nothing that Spettigue or anyone else can do. All is forgiven, and everyone (except for Mr. Spettigue) is happy ("My Darling, My Darling – Reprise").