A prim and proper lady has an alter ego that comes out while she sleepwalks.

Prim nurse Nora Gilpin reacts disparagingly upon learning that wealthy lawyer John Raymond, Jr. will be attending that day's hospital board meeting, as she believes that his philanthropy is self-serving. As Nora goes about her business, her subconscious mind tries to remind her of her childhood love for John, but her conscious mind drowns out the musings and also perpetuates her amnesia of their past as playmates. Determined to rid herself of any nagging doubts that she has unfinished business from her past, Nora accepts the marriage proposal of her longtime beau, contractor Tim McCarey. Nora's boss, kindly Dr. Jackson, is worried about Nora, but her father, botanist Harry Gilpin, accepts her engagement when she insists that she is happy. After her engagement party ends, however, a sleepwalking Nora rises from her bed, dons a sexy dress and goes to John's mansion. There, she flirts with the stuffy John, calls him a frog and kisses away the "warts" caused by his priggishness. John is bewitched by Nora, whom he does not recognize even though some of her mannerisms are familiar, and becomes frustrated when she refuses to divulge her name. After spending the night talking, Nora slips away and goes home, where she returns to bed. Shortly after, Nora awakens and is bewildered by her exhaustion. Upon finding the dress and a cigarette butt, the non-smoking Nora asks Jackson how she can help a "friend" who apparently has been sleepwalking. Realizing that Nora is talking about herself, Jackson states that sleepwalking can be caused by a deep frustration that splits the unconscious mind into a separate personality in order to satisfy its desires. A week passes as a private detective hired by John tries to locate his mysterious visitor without success. One day, as John wanders the streets, he spots Nora and Tim browsing in a furniture store. John grabs Nora and calls her his princess, as she had referred to herself during her visit, but Nora does not remember their night together and rebuffs him. Baffled, John accepts the explanation of his partner, Michael Hogan, that he mistook Nora for his mystery woman because he is so obsessed with her. Later that night, John is about to board a train for Washington, D.C, where he has an important hearing, when the sleepwalking and sexy Nora appears and asks him to stay. As they later relax on the beach near an amusement park, John remembers going to an amusement park long ago with the young daughter of the gardener who was landscaping the Raymond estate. John finally realizes that Nora was the little girl and laughs when he remembers that he used to call her "Jughead." The couple then try out the park's roller coaster and have so much fun that John pays the attendant to stay open past closing time and they ride on it for hours. The next day, a newspaper prints the story of John's "midnight prank," while Nora, still unaware of her sleepwalking, exhaustedly prepares for her wedding to Tim. John comes to the hospital to visit Nora, but she again denies being involved with him, and John creates such a fuss that he is arrested. John decides that he wants to take the case to court, to force Nora to admit that she knows him, but during the trial, Nora cannot remember anything when John asks her about their nocturnal visits. She is surprised to see a piece of lace from her petticoat, which was torn on John's garden gate, but still denies John's assertions about their romance. Nora's primness then convinces John that he has indeed mistaken her for his dazzling "Jughead." Meanwhile, Nora rushes home and matches the piece of lace to her torn petticoat, and Jackson advises her to postpone her wedding until she can sort out her feelings for John. Still declaring that she hates John, Nora consumes coffee and plays the radio loudly to prevent herself from falling asleep and unconsiously visiting him. John, who has gotten drunk, waits outside Nora's house and, upon speaking to the neighborhood policeman, learns that Nora sleepwalks, as did her late mother. John then consults with Jackson, who warns him that Nora will never be happy unless her conscious personality meshes with "Jughead." Still wanting to marry Nora, John is uncertain what to do, while at the Gilpin home, Nora falls asleep and, sleepwalking, takes a packed suitcase to meet John. Nora seductively asserts that she is marrying him rather than Tim, and John hurries her off to a justice of the peace before she wakes up. In the morning, Nora awakens to find herself in a motel room with John, and sneaks out without waking him. Horrified that her nocturnal wanderings have turned illicit, Nora is distraught when she returns home but determines to marry Tim. During the ceremony, Nora keeps dozing off until John bursts in, declaring that she is already married to him. Nora faints, but when she recovers, she finally realizes that she loves John, and rewards his patience with a lovely smile and a big kiss.