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Mary Hale, the American authority on child psychology, returns from a trip with her daughter, the prim genius Stephanie, and tells reporters about her plan to help Senator John Pringle with his work on the youth problem. One of the reporters, Jeff Turner, happens to be Mary's ex-husband and Stephanie's father, and accuses Mary of sacrificing her marriage for her career and stepping into a man's job. Soon after, Mary asks Jeff if she can visit Stephanie's identical twin sister, Terry, whom she has not seen for seven years, and he agrees. That night at the soda shop, "bobby-soxer" Terry, her boyfriend Jimmy and their friends discuss the City Championship Dance Contest before she comes home to find her mother and sister. As Mary and Jeff discuss the fact that though they still care for each other, they disagree on how to rear the twins and whether Mary should work, Terry and Stephanie covertly decide to switch places, so Terry can enjoy Stephanie's glamorous lifestyle and live with her mother, while Stephanie can escape the public eye and get to know her father. When Jeff refuses to let Mary take Terry and instead kisses her, a flustered Mary grabs Terry, thinking she is Stephanie, and leaves. The next morning, Stephanie visits the soda shop and confuses Terry's friends by being awkward and brainy. At the same time, Terry goes out with Stephanie's boyfriend, Mickey Pringle, and shocks him by being fun and wild. That night, the girls talk on the telephone and decide that Stephanie should compete in the National Knowledge Contest, dressed as Terry, so their mother will believe that Jeff reared Terry correctly and fall in love with him again. Soon after, Stephanie is forced to compete in a jitterbug contest with Jimmy, and though she fails miserably, the two fall deeply in love. Meanwhile, Terry attends a society dance and falls in love with Mickey, who adores what he believes is his girl friend's new liveliness. A few days later, after Mary gives a public speech supporting Pringle, whom she plans to marry in order to advance her career, Jeff maneuvers her into his cab. There they admit their mutual love, though Mary refuses to re-marry him. Later, after Jeff stands up Alice, his jealous girl friend and fellow reporter, to cover the National Knowledge Contest, she gets even by photographing Terry, as Stephanie, dancing the jitterbug with Mickey and getting involved in a fight that breaks out on the dance floor. Meanwhile, Stephanie, as Terry, wins the National Knowledge Contest, causing Mary to change her opinion about Jeff's parenting skills. Alice calls Jeff at the award ceremony to inform him that Stephanie is in jail for fighting. Leaving Mary at the ceremony, he runs to the newspaper office, where Alice blackmails him into agreeing to marry her in return for not printing the photographs of Stephanie jitterbugging, which would ruin both Stephanie and Mary. When Mary, who has followed Jeff, arrives at the newspaper office, she tells him she now wants to marry him, but he feigns disinterest. A few days later, Terry forces Jeff to tell her about Alice's blackmail, and after she and Stephanie reveal their identity-swapping to their friends, the teens help them storm Alice's apartment and steal the pictures. That night at a rally, Mary is about to announce her engagement to Pringle when the kids burst in and give the photographs to her. As Mary tears up the pictures and embraces Jeff, Alice storms out and the twins swap boyfriends, kissing them blissfully.