The musical starts with the busybody postman who reads everybody's mail, Hector, delivering mail to the Dana household and particularly to Cora, the maid he is in love with. Professor Oliver Dana is the head of the household. The oldest sister is Sylvia, an actress, and the youngest is Nancy, who is a bit of a flirt and has all the boys fighting over her. The middle sister Jane, the "nice girl", makes her entry singing the song "Perhaps" to the rabbits she takes care of. Her father is trying to write a book about diet and is testing it on rabbits. Don Webb is her boyfriend, an avid car buff.
One day, Richard Calvert visits the Danas to study what Professor Dana is practicing with the rabbits. The three daughters are instantly enamored with him and do all they can to impress him. The family gets together with Jane playing the piano and singing "Beneath the Lights of Home", accompanied by Oliver, Nancy, and Sylvia.
There is a Fourth of July celebration in which Oliver Dana gives a speech. After a bit of dancing, Jane sings "Old Folks at Home". Don lends Jane the car to take Richard to the train station, but she decides to stop being a "nice girl" and drives him to New York. When they get there, she changes into some clothes that belong to Richard's sister, and then plays a song on the piano, "Love at Last", while she sings, and Richard comes down and sees her at the piano. Richard quickly sees that she is trying to not be a "nice girl", but both of them realize that she is just playing a game and not really being who she is. So, she leaves his house and drives back home, still wearing Richard's sister's pajamas. She gets into town, but the car runs out of gas, and she manages to wake up the entire neighborhood. They all see that she is no longer a "nice girl", and rumors start to fly instantly. Jane locks herself in her room until her father gives her a telegram from Richard.
Jane decides to face the music and go to the benefit for the International Red Cross and Red Crescent Movement, but doesn't realize that everyone thinks that she and Richard are engaged. She sings "Beneath the Lights of Home" at the benefit. Everyone congratulates her afterwards about her engagement, but she thinks they are just fooling her. She goes to Don for comfort, and he agrees that it's all nonsense, and, of course, Richard wouldn't go for her. Jane gets angry at that and decides to show Don. She tells him that she is engaged to Richard, and Don storms off. Right at that time, Oliver and Richard come to the benefit. They congratulate Prof. Oliver (for the engagement of his daughter to Richard Calvert), but he is confused and thinks that they are congratulating him for getting his fellowship. Jane intercepts them and tells Richard that everyone thinks they are engaged. They fake a big quarrel so that they can break their engagement.
Meanwhile, Don has enlisted in the army, and Jane goes to see him to explain what happened. When she goes, Don tells her that he loves her. They kiss (off-screen), and then she goes to a bandstand and sings the song "Thank You America" for the crowd at the Army base.
