Marcia Bradburn, who is completing her last year at Emma Hope's finishing school, is overjoyed that her father has given permission for her to become engaged to Roy Todwell. When she meets Roy in the school garden, he begs her to elope with him and becomes sullen when Marcia insists that they wait. Then suddenly, Roy throws a rock at his cocker spaniel and kills it after the dog wimpers at a train whistle. A sobbing Marcia goes to Emma, who calls her old friend, wheelchair-bound Dr. Leonard Gillespie of New York City's Blair General Hospital. Gillespie summons staff member Dr. John Hunter Gerniede, a surgeon who has recently arrived from Holland and wants to transfer to psychiatry. On Saturday evening, Gillespie and Gerniede go to a dance at the school so that Gerniede can surreptitiously observe Roy. Gerniede asks Marcia to take Roy to the spot where he killed the dog, and sees that Roy does not remember the incident. A few days later, after the boy has been examined at Blair, Gillespie and Gerniede tell Roy's parents that he has a serious mental illness that needs treatment. Because their family physician, Dr. Ward O. Kenwood, thinks that Roy has merely been working too hard at school, his parents refuse to accept Gillespie's diagnosis and take him home. That same afternoon, Marcia and Roy go shopping and while staring at a model train set in a store window, he becomes violent and tears the display apart. Marcia then goes to Dr. Gerniede and tells him what happened. Kenwood and Roy's parents still do not accept Roy's illness, but Marcia resolves to help him. At home, Roy imagines that Gillespie is against him and only pretends to take the sleeping medicine that Kenwood gives him. He then gets dressed and flees his room. Some time later, when Blair nurse Parker finds an unsigned, threatening postcard addressed to Gillespie, she shows it to hospital chief Dr. Walter Carew. Carew consults Gerniede, who thinks that he knows what is causing Roy's problems and, after calling Marcia, determines that loud piercing noises, such as train whistles, trigger Roy's violent outbursts. Hoping to keep the threats from Gillespie, Carew assigns muscular orderly Joe Wayman to act secretly as Gillespie's bodyguard. Meanwhile, in Detroit, Roy takes up with a dancehall girl named "Bubbles." Hoping to impress her, Roy arranges to purchase an expensive automobile C.O.D. The dealer and his mechanic then go to the address that Roy has given them, an unoccupied house, and Roy kills them both. That evening, at a restaurant, Roy goes to the men's room and hears a piercing sound, then walks out of the restaurant in a daze and kills a policeman. Some time later, when Marcia and Emma see Roy outside the school, they telephone Gerniede, who suggests that for her own protection, Marcia stay in his place at the hospital. After locking Marcia in his apartment, Gerniede discovers a broken picture of a train and knows that Roy is nearby. Although Carew initiates security procedures, Roy poses as a doctor and is able to get into Dr. Gillespie's unoccupied offices. When Gerniede goes to the office, he finds it empty, but discovers that a large scalpel is missing from a cabinet. He then goes to check on Marcia, who is not in the apartment. Meanwhile, in one of the hospital laboratories, Marcia, whom Roy had telephoned, approaches and begs him to call Gerniede. Although Roy says that he will kill Gillespie, he lets her call Gerniede to tell him where they are. When Gerniede, Gillespie and the police arrive, Gillespie convinces the police to let him and Gerniede talk privately with Roy. While pretending that he wants help, Roy secretly grabs a gun that he had earlier hidden in Gillespie's desk and is about to shoot when Joe enters the room and knocks the gun away. Several months later, Gerniede learns that he is being transferred to the psychiatry department, and Gillespie assures the graduating Marcia that Roy, who is now in prison, did not know what he was doing.
