Three daughters try to stop their divorced father from marrying a gold-digger, while one of them falls in love with the wealthy man used as bait.
On a lake in Switzerland Penny Craig (Deanna Durbin) sings "My Heart Is Singing." She and her older sisters Joan (Nan Grey) and Kay (Barbara Read) find their divorced mother upset because their father may marry again. Penny suggests that they go to New York, and housekeeper Martha provides the money. Judson Craig (Charles Winninger) is trying to lose weight and learns that his daughters are coming. Judson lunches with Donna Lyons (Binnie Barnes), and the three girls join them. Donna leaves and complains to her mother Mrs. Lyons (Alice Brady), who welcomes them at dinner. Penny is sent to bed, and Donna sings "Someone to Care for Me" flatly. Judson complains Penny made noise with her bed. He tries to fix the bed, and they laugh. Bill Evans (John "Dusty" King) advises the girls to find someone richer for Donna, and Penny sings to her father ""Someone to Care for Me."
Evans wakes Count Arisztid (Mischa Auer) and hires him to court Donna. At a nightclub Arisztid drops the magazine Evans gave him, and Michael Stuart (Ray Milland) picks it up. Thinking he was hired, Kay brings him to their table. Michael dances with Kay, who tells him to court Donna. Michael dances with Donna and gives her his phone number. Kay calls Michael, who says he needs money to take Donna to lunch. Kay borrows from the butler. Michael buys a ring for $7,000. Kay asks Michael about Donna and gives him money. He tells her he may marry Donna for her father's sake. Michael shakes Kay and takes her key, staying with her while Donna waits. Arisztid calls on Evans, who is pleased, and Arisztid gets $500. Judson takes roses to Donna and asks her to marry tomorrow. Newspapers report it. Evans learns that Kay gave money to the count and comes back after fighting him. Worried Kay goes to Michael, who doesn't know Evans. Kay learns who Michael is, cries, and runs out. Michael calls Donna and gives her tickets to Europe. Penny wakes her father and says she can't bear to see him marry Donna. Judson finds Kay gone, and police take her to a judge. Penny says that she is an opera star and sings "Il Bacio." Donna and Mrs. Lyons complain about Penny's tricks and insist Judson marry that day; but he refuses. Mrs. Lyons says they are sailing with Michael and find Arisztid aboard but not Michael. Police bring Penny to Judson, and they are happy; but Kay cries and runs into Michael, who kisses her. In the final scene the family welcomes Dorothy Craig at the dock.
This film is significant for helping to begin a trend of movies about teenagers. Their energy and good intentions are expected to help the adults solve their problems, and in a comedy they usually do.