Fed up with New York life after one year's residence, Susan Applegate decides to use the money that she has saved for her return train ticket home but discovers at the ticket window that the fare has gone up. Unwilling to remain in New York a day longer, Susan gets an inspiration when she sees a child receive the ticket at half-price. She dashes into the women's bathroom, rubs off all her makeup and dresses as if she were a little girl. She then pays a stranger to buy her a child's ticket on the train and maintains the ruse even after she boards. When suspicious conductors catch her smoking a cigarette, Susan hides in the sleeper cabin occupied by Major Philip Kirby, a military boys' school instructor who is returning from Washington, D.C. after trying to get his military status reactivated. Philip, taken in by "Su-su's" childlike behavior, shelters her from the conductors and lets her stay the night in the lower berth. The next day, the train is detained by flooding on the tracks, and Philip's fiancée Pamela and future father-in-law, Colonel Hill, drive up to meet him. When Pamela sees Susan in Philip's room, she thinks he has been having an affair, and Philip is forced to bring Susan to the military school to prove that she is only a twelve-year-old child. After the misunderstanding is cleared up, Philip insists that Susan remain at the school for a few days until someone can escort her home, and she stays at the Hill home. Pamela's teenage sister Lucy immediately sees through Susan's get-up, but because she thinks her sister is a "stinker," she befriends Susan. Lucy reveals that although Pamela claims she is helping Philip fulfill his wish to get active duty, she actually has been using all her connections to prevent him from enlisting so that they can remain at the academy after they are married. Susan becomes popular with all the young boys at the school and has to fend off their amorous advances. After witnessing Pamela's subterfuge, Susan tricks one of Pamela's connections into getting Philip reinstated, but when Pamela hears the news, she calls off their engagement. Pamela learns of Susan's involvement and forces her to leave by threatening to expose her and Philip in a scandal. Susan, who has fallen in love with Philip but has been unable to reveal her true age to him, reluctantly returns home to her mother. Philip stops to see her on his way to California to report for duty, and Susan pretends to be her mother, allowing Philip to leave without learning the truth. However, after hearing that Pamela has married someone else, Susan changes into her own clothes and rushes to the train station to join the finally enlightened Philip on his journey, and they plan to marry en route to California.