Act I

After a rousing opening number (“I Love a Musical Comedy Show”), the lights come up on a lovely Milwaukee spring morning in 1912. Harry and Jack, two handsome students, have met the girls of their dreams and intend to propose marriage that very day (“I Want a Girl Just Like the Girl”). They quickly get off an invitation to Katy and Margie under the guise of meeting Harry’s rich aunt from Brazil. To help distract old Auntie, the boys ask their music professor Babbs to lunch as well (“Look for the Silver Lining”). Babbs agrees and then uses Jack's room to try on a costume for a amateur production of The Importance of Being Ernest in which he plays Lady Bracknell.

Katy and Margie arrive, and the two couples recall the night they met (“By the Light of the Silvery Moon”). But, as Auntie has not yet arrived, the girls leave, promising to return. Annoyed, Jack orders Harry to go fetch his aunt. While waiting for Harry to return, Jack’s father, Frank Chesney, suddenly arrives with bad news. They are broke. Horrified at the thought, Jack suggests that his father could marry Harry’s rich aunt. Frank is hesitant, but agrees to come back and meet her.

Harry returns with more bad news. His aunt won't be coming after all. At first panicked since the girls won't stay without a chaperone, Babbs enters dressed as Lady Bracknell and saves the day. The boys convince a hesitant Babbs to pose as Harry’s aunt (“Baby Face”). When the girls return, they are introduced to Babbs as "Auntie" and they instantly adore her/him.

Jack’s father returns, but before lunch can start, Margie’s grumpy uncle, Mr. Spettigue, arrives and is furious that the girls are there without his permission. When he learns that Harry's aunt is rich, both he and Frank stay and try to impress her for obvious financial gains (“Oh, You Beautiful Doll”).

After lunch, Jack proposes marriage to Margie and she happily consents (“My Little Margie”). In the meantime, Frank decides to woo "Auntie," much to Babbs' consternation (“Runnin’ Wild”). To complicate matters further, Harry’s actual aunt, Donna Lucia, arrives after all with her companion, Ida. Ida reminds Donna Lucia of how she met and fell in love with a man named Babbs. Donna Lucia also shares a story of her own lost love (“My Buddy”). When Frank enters, both he and Donna Lucia are thrilled to realize that they were sweethearts years ago. But when Donna Lucia discovers that someone is impersonating her, she quickly makes up a false name and plans on discovering the reason for the deception.

After Harry finally makes his declaration of love (“K-K-K-Katy”), Jack and Margie join the happy couple to share in the good news ("I’m Just Wild About Harry"). Unfortunately, the girls need Spettigue's consent to marry. They decide to ask "Auntie" to help them. Everyone converges on Babbs to get what they need, but poor "Auntie" is exhausted and stops the show, declaring it’s time for an intermission.

Act II

After Frank shares his feelings for Donna Lucia (“If You Were the Only Girl in the World”), the girls rush in with Babbs and try to convince "Auntie" to help them (“Abba Dabba Honeymoon”). Babbs agrees to try and get a letter of consent from old Spettigue, but before he can, he is confronted by the real Donna Lucia, who tries to figure out just who this odd woman is. Babbs is clearly shocked when Donna Lucia introduces him to Ida, a girl he fell in love with several years before. Ida explains to him that she also has a secret love (“I’m Always Chasing Rainbows”).

Babbs resolves to end the charade and reveal himself to Ida but must convince Spettigue to allow the girls to marry first. He dodges Donna Lucia’s questions by singing a fake Brazilian song (“Spanish Love”) and then, while the young lovers hide, he tricks Spettigue into giving him a letter of consent for the marriages (“Under the Bamboo Tree”).

That done, Babbs turns his attention to Ida who hints that she might in fact know who he is (“You Made Me Love You”). Babbs finally admits that he is not Harry’s aunt, but the man she loves and runs off to change. Frank and Donna Lucia also make their own wedding plans (“Daisy Bell”).

Babbs, now dressed as himself, admits to everyone that he had been playing the part of Harry's aunt. He tries to return Spettigue’s letter of consent when the real Donna Lucia reveals herself, shocking everyone. She takes the letter, declaring it is legal.

Spettigue fumes, threatening to dispute the letter, but Donna Lucia convinces him to agree to the couples marrying by introducing him to her even wealthier friend, Donna Frederica De Sanchez, also from Brazil. Happy with the arrangement, Spettigue agrees to the marriages. With all the couples paired off, they celebrate their good fortune (“For Me and My Gal”).