A mother and daughter compete over the same singing role and, unbeknownst to each other, the same man.
On the closing night of a Broadway play, leading actress Frances Elliott (Ann Sothern) hosts a party attended by many guests, including her eccentric father Gregory (Louis Calhern), who is also an actor; her 17-year-old daughter, Nancy Barklay (Jane Powell), an aspiring actress; and Brazilian playwright Ricardo Domingos (Fortunio Bonanova), who is considering starring Frances in his next play.
Frances eagerly pursues the part in Ricardo's play, and though she is virtually assured of the role, Ricardo asks her not publicize the news until a final decision is made. Later, Ricardo privately tells Frances' producer that Frances may not be right for the part and that he had a younger actress in mind. Then, when Ricardo meets Nancy, he instantly knows that he has found the perfect young woman for the role.
The next day, Frances sets sail for Rio de Janeiro, where she intends to vacation and devote herself to studying her lines. Gregory accompanies Frances to Rio, while Nancy, who is about to star in a small stock company play, goes to Connecticut. After observing Nancy's acting abilities, Ricardo offers her the part that he promised Frances. Nancy accepts the role, though she is unaware that Ricardo has already promised it to her mother.
Seeking the quiet she needs to study for the part, Nancy follows her mother and grandfather to Rio. On board the ship, businessman Paul Berten (Barry Sullivan) overhears Nancy rehearsing her lines and mistakenly concludes that she is a deserted wife and an expectant mother. Paul takes pity on Nancy and enlists the help of his business partner, Marina Rodrigues (Carmen Miranda), to counsel the young girl.
Nancy does not know that Paul is trying to help her and mistakes his paternal concern for a marriage proposal. She rejects Paul's apparent proposal, and bids him farewell when the ship reaches Rio.
Soon after she is reunited with her mother, Nancy overhears her rehearsing her lines and immediately realizes that they are studying for the same part. The revelation devastates Nancy and prompts her to bow out of the play. She does not tell her mother that she was set to star in Ricardo's play, and instead informs her that she is in an entirely different play.
Confusion abounds when Nancy later visits Paul at his office and tries to accept the marriage proposal she thought he had made. Paul is perplexed by her behavior, and still thinks that Nancy is pregnant and troubled. He sends her home to talk to her mother about her situation, but Nancy misunderstands him and thinks that he meant for her to discuss their impending marriage with her mother.
Marina follows Nancy to her mother's house, and privately tells Frances about Nancy's supposed pregnancy. The confusion is heightened when Frances misunderstands her daughter's anguish and concludes that she must be pregnant by Paul.
Frances demands a private meeting with Paul, during which he reveals his romantic attraction to Frances. Frances leaves Paul in disgust, but the situation is soon clarified when Paul tells Gregory that he had only just met Nancy on the boat. Gregory immediately recognizes Nancy's supposed predicament from the story of the play that Frances was reading, and explains the situation to Frances.
When Frances learns the truth about Paul, she changes her impression of him and they embark on a romance. After announcing her engagement to Paul, Frances withdraws from Ricardo's play and suggests Nancy as her replacement. All ends happily when the show opens in New York with Nancy in the starring role.
