In the summer of 1913, seventeen-year-old Patti Robinson joins her mother Katherine, her father Horatio, her younger sister Melba and her younger brothers McCormick and Ricky on her family's annual vacation at a mountain resort in Kissamee, New York. Though Patti believes that she has reached adulthood and is ready to begin dating boys, her somewhat prudish and over-protective mother believes otherwise and insists that Patti wait until she reaches the age of eighteen. As soon as Patti arrives at the resort, Billy Finlay, the sixteen-year-old son of the hotel manager, resumes his objective of the previous summer and tries to court her. Patti avoids Billy and instead turns her attention to an unfamiliar guest at the resort, a handsome young Cuban man named Demi Armendez. Demi also catches the eye of Patti's friend Valerie Stresemann, who is over eighteen, egotistical and wears a corset to pinch her figure. Patti wishes that she, too, could wear a corset to attract young men, but her mother insists on outfitting her with clothes to make her appear young and, to Patti's mind, unattractive. One day, during a family outing at the riverbank, Patti sees Valerie and Demi approaching and, embarrassed to be seen playing on the children's side of the beach, asks Melba to bury her with sand. Horatio later learns that Patti has been ridiculed for wearing an ugly bathing suit, so he takes his daughter's side and urges Katherine to show more consideration for Patti's feelings. Later, Valerie tries to sabotage Patti's attempts to attract Demi by giving her bad advice and telling her that she should act more like the famous motion picture actress Theda Bara. Patti tries to act more worldly, mysterious and helpless, but her plan fails and she eventually realizes that Valerie is not a true friend. One evening, while canoeing on the river, Patti falls asleep and has a dream. In the dream, Demi compliments her singing and professes his love for her but reacts with outrage when she reveals that she is not wearing a corset. Patti awakens from her dream screaming for Demi to return and falls into the lake when she tries to stand up in the canoe. Demi sees Patti fall into the water and tries to save her, but she saves herself and runs off into the woods, embarrassed again. Patti returns to her room soaking wet, and when Demi follows her there, Horatio misunderstands his motives and accuses him of improper behavior. Later, when Patti learns that Demi is taking Valerie to the big dance at the resort, she becomes despondent. Refusing to attend the dance, Patti locks herself in her room and studies Spanish instead. Patti eventually agrees to attend the dance with Billy, and, much to her surprise, she winds up in Demi's arms when he asks her to dance. She returns to her room later that night feeling happy and lovesick, but she is still angry that she is not allowed to wear a corset. In an attempt to help Patti's difficult transition into womanhood, Horatio goes to town to buy her a corset. Unfortunately, Horatio's lack of knowledge of women's undergarments results in his purchase of a surgical corset. Unaware that her corset is a surgical one, Patti wears it during her dance performance at the resort talent show. While dancing a tango with Demi, she is dipped and her corset locks. Unable to straighten her back, Patti is rushed backstage, where she is released from the corset's hold with help from her mother, who finally realizes that her daughter has grown up. The summer vacation ends happily for Patti when she makes amends with her mother, and when Demi promises to call her.