Energetic New York teenager Joan Lyons is the president of Guiding Stars, Ltd., a high school fan club, whose members are required to collect four movie star autographs every month. As soon as Joan and her best friend, Patricia "Patsy" Drew, learn that Greer Garson has just arrived in town, they track her to her hotel. There Joan tries to trick her way up to the star's room by posing on the phone as Louella Parsons. Garson overhears Joan's impersonation and, to the girls's astonishment, invites them to her room. Over tea, the girls interrogate the ever-polite Garson and are further delighted when Walter Pidgeon, Garson's popular co-star, shows up. Pidgeon is also kind and patient with Joan and Patsy and happily signs their autograph books. When Joan finally returns home, her father, tax lawyer Burton V. Lyons, is furious with her for being late and refuses to believe her story about Garson and Pidgeon. As punishment, Burton orders Joan to stay home that night, but after he and her mother Edith leave for the evening, Joan, her boyfriend Schuyler and her garrulous younger brother Junior go to the movies. To their dismay, Burton is waiting for them when they return and informs Joan the next day that her allowance has been cut off. Junior's meddlesome governess, Miss Featherstone, is also in trouble with Burton and receives a lecture from him in his office. Afterward, she sees his secretary, Susan Thayer, model a lacy nightgown for him and, unaware that Susan is in love with Burton's shy associate, Douglas Sutton, and the nightgown is an anniversary present for Edith, assumes the worst. Later, during a Guiding Stars meeting at her apartment, Joan learns from new member Vera Bailey that Robert Taylor is in New York. While Joan is calling hotels to find out where Taylor is staying, Susan is trying to call the Lyons' home to tell Miss Featherstone to pack Burton's suitcase for a brief business trip. Unable to get through, Susan telephones the neighbor's apartment, and her call is answered by Taylor, who happens to be staying there. Unaware to whom she is speaking, Susan asks Taylor to deliver the message to the Lyons', and he shocks the Guiding Stars when he appears on Joan's doorstep. Later, Susan shares a taxicab with Doug, who finally confesses his love and proposes. After the now ecstatic Susan drops off some flowers and a letter for Edith from Burton, which explains why he cannot give her an anniversary present until he returns, Miss Featherstone destroys both the flowers and the letter. Miss Featherstone then tells Joan that her father is having an affair with Susan. Concerned, Joan and Patsy follow Susan to the train station and see her father give Susan a congratulatory hug before he boards the train. With her worst fears seemingly confirmed, Joan goes to see Walter Pidgeon for advice. Although Pidgeon counsels Joan to stay out of her parents' affairs, Joan decides the only way to save their marriage is to make her father jealous. To that end, she hires Italian strong man Hercules to impersonate a diplomat at a charity ball that her parents are attending on their anniversary and flirt with her mother. When she discovers that she doesn't have enough money to pay Hercules or rent his tuxedo, Joan sells her beloved autograph book to Vera for $25. Joan then persuades her unsuspecting mother to try a new, more glamorous look. At the ball, the crude but persistent Hercules, whom Joan introduces as "Dr. Hercules," succeeds in making Burton jealous and provokes him to fight. After he is knocked down, Burton notices the rental label on Hercules coat and accuses him of being an impostor. Hercules then reveals all to Burton and Edith, and when Burton discredits Miss Featherstone's assumptions, a shamed Joan runs away. At the Lyons', Burton confronts Miss Featherstone, who admits her deeds and is fired. Later, with Patsy's help, the Lyons' deduce that Joan has become a Salvation Army volunteer. After a tearful reunion, Joan returns home with her parents, who have bought back her autograph book from Vera, but is so tired that she fails to notice that William Powell is riding in the elevator with her.