Susan Applegate, a frustrated young woman working as a scalp massage therapist in New York City, decides to quit her job and return home to Iowa. Constrained by her limited finances, she resorts to masquerading as a 12-year-old girl to buy a half-price train ticket. Her disguise works, but complications arise when she is discovered by a suspicious conductor. In an attempt to avoid getting caught, she ends up in the care of Major Philip Kirby, an unsuspecting military school instructor who believes her ruse and offers his protection.

As Susan continues her charade, she navigates a series of comedic and precarious situations at the military academy where Kirby works. Pretending to be his niece, she interacts with the students and faculty while trying to maintain her disguise. Meanwhile, Kirby's fiancée, Pamela, becomes increasingly suspicious of Susan's true identity, leading to further complications. Just as tensions mount and secrets teeter on the brink of revelation, Susan's underlying motivations and personal conflicts come to the forefront, setting the stage for unexpected resolutions and heartfelt moments.