Carrie (Diana Barrymore), ambitious young actress and her manager Mike Kilinsky (Andy Devine), travel east from Chicago to Long Island so Carrie can see the new mansion that her mother Chris (Kay Francis) has just purchased. Chris is not there when Carrie arrives by Gallagher (Ethel Griffies), the family major domo, tells Carrie that Chris is in love with the very rich Steve (John Boles.) Chris confesses to Carrie that she has lied about her age to Steve, claiming to be 31, and that the photograph of Carrie is Carrie's (non-existent) Aunt Sadie and Carrie is only twelve. Carrie insists that her mother keep up the masquerade, and she will pose as a 12-year-old. Steve arrives and brings with him his friend Jimmy Blake (Robert Cummings). More than a little confusion follows including Carrie starting a fight with another "kid" to keep Jimmy from keeping a date with his fiancee and Mike escorting Chris and the "under-age" Carrie to a night club, and other incidents that only tend to prove that Steve and Jimmy aren't the brightest bulbs shining. Diana Barrymore is better as the 12-year-old than she is later playing Joan of Arc in a Detroit theatre, where the light finally dawns on the perplexed Robert Cummings character.

After finishing a stage performance in Detroit, actress Caroline Bishop heads east to see the Long Island mansion that has just been purchased by her wealthy, widowed mother Christine. Arriving at her mother's new home, Caroline is surprised by the flashiness of the decor, then is told by the old family retainer, Gallagher, that her mother is in bed and has been out "lallygagging" until three in the morning. Christine tells Caroline that the change in her lifestyle is due to the new man in her life, multi-millionaire Steven J. Forbes. Christine further admits that she has lied to Steven, having told him that she is only thirty-one years old and that her daughter is a mere child away at boarding school. She has also told Steven that a photograph of the adult Caroline is "Aunt Sadie," the "black sheep" of the family, whose picture she keeps as an "object lesson" for Caroline. When Steven comes to call on her the next afternoon, Christine plans to tell all, but Caroline insists that her mother continue the deception and disguises herself as a twelve-year-old. Steven arrives at the Bishop home with Jimmy Blake, a youthful millionaire, who has devoted his life to fighting for the downtrodden and who believes that Caroline is being mistreated by being forced to attend boarding school. When Gallagher threatens to quit the Bishop household unless Caroline and Christine tell Steven and Jimmy the truth about their ages, Caroline locks the servant in a closet. Jimmy returns the next day and surprises Caroline while she is dressed in her "Sadie Thompson" costume, rehearsing for her next play. Jimmy mistakes this "adult" Caroline for Aunt Sadie and attacks her, thinking that she has been mistreating the "child" Caroline. The "child" Caroline later comes down the stairs feiging injury, and falsely claims it is the work of her evil aunt. Having fallen in love with Jimmy, Caroline tries to keep the young millionaire from a date with his fiancée by telling him that she has never tasted ice cream. Once at the soda fountain, Caroline gets into a fight with a twelve-year-old boy, who then summons his teenage brother and ex-prizefighter father for help. Fleeing the situation in Jimmy's car, the two are arrested for numerous traffic violations, and Christine is forced to go to the police station to bail them out. Later that night, Jimmy discovers the truth when he sees the adult Caroline at a nightclub with Christine and Mike Kilinsky, Caroline's theatrical manager. Although Caroline tells him that she is Sadie, Jimmy recognizes the bandage on her injured wrist, a result of the soda fountain escapade. Caroline persists in her deception, however, by insisting that the "child" Caroline is still at home, then is forced to race back to the Bishop mansion before Jimmy's arrival. The millionaire then gives the "child" Caroline the punishment he feels she so richly deserves by throwing her into a lake. Later, Steven proposes marriage to Christine, and Jimmy traces Caroline to a Detroit theater, where she is appearing in a play as "Joan of Arc." Jimmy confronts Caroline with the truth, and in the midst of her perforance, he proposes to her and she accepts.