A girl vocalist and her apartment manager get mixed up in a creepy Park Avenue murder and find themselves facing danger at every turn.

Waldo Williams, the building manager of a Park Avenue apartment house, would rather practice his drums in the basement than deal with the complaints of the fussy, rich tenants. One morning he discovers eccentric tenant Travis Revercombe dead, and has just called the police when Benjamin Goodman, Revercombe's attorney, arrives and instructs Waldo to send for Dr. Glenson, another tenant. Glenson says that Revercombe, who suffered from heart trouble and diabetes, appears to have died of an overdose of insulin. The medical examiner arrives and corroborates Glenson's opinion, and the men move the body into the bedroom. When the undertaker arrives, however, he discovers a carving knife stuck in the corpse's chest. Revercombe's estranged niece, singer Jody Revere, arrives that evening, and takes the news of her uncle's death coldly, telling Goodman that Revercombe had once propositioned her mother. Goodman tells Jody that her uncle left her the bulk of his estate, but she refuses to take the money. Music lover Waldo is thrilled to meet Jody, and as they sing and dance to one of her recordings, she is impressed with his skill as a drummer. Jody decides to stay in Revercombe's apartment while she tries to contact her bandleader, PeeWee, who is on the road. While Jody is in the bedroom changing clothes, a secret panel in the wall opens and an elegant woman enters and observes her unnoticed for a moment. The next morning, Goodman again urges Jody to accept her inheritance, saying that the money will otherwise go to a language research organization run by Valeska Chauvel, who is coming over to meet her. Valeska, who is the same woman who entered the bedroom the previous day, tells Jody about her plan to develop a universal language, but Jody is not impressed and decides to take her inheritance and give it to the American Guild of Variety Artists. After Valeska leaves in anger, homicide detective Lt. O'Flair brings the news that preliminary autopsy reports on Revercombe indicate foul play. O'Flair questions the doorman, ex-convict Ben, who says that tenant Kyle Manning, an artist whose recent exhibition had been shut down by the prudish Revercombe, was seen near the apartment and left town suddenly that morning. Unnoticed by O'Flair and the others, Valeska sneaks back and listens through a secret panel as Jody describes her tendency to sleepwalk whenever she hears music. That evening, Waldo plays his drums for Jody, and when PeeWee calls with the news that the band's drummer has been drafted, she has Waldo audition for him over the phone. Later, as Jody sleeps, Valeska enters the bedroom carrying a music box, which she uses to lure the sleepwalking Jody onto the balcony. Before Valeska can push Jody over the railing, however, Waldo plays a record in the living room and Jody wanders away. Realizing she is asleep, Waldo declares his love and proposes to Jody. After she returns to bed, Waldo goes to the building office and inspects the blueprints for the apartment. He then enters the deserted apartment in the building next door, and while searching for hidden passageways, discovers Valeska's body in a closet. At that moment, Jody hears a sound and finds Manning in the living room, trying to warn her. Waldo hears her scream, and rushes in after she faints. He leaves her with Dr. Glenson, who is preparing a sedative when the police call Jody to warn her about him. The sinister doctor then confesses that he tricked Revercombe into believing he had diabetes so that he could arrange an overdose of insulin. Glenson adds that he was in cahoots with Valeska, who was about to leave him and run away with Revercombe. Glenson then killed Revercombe, but Valeska got even with him by putting a knife in the corpse. He is about to kill Jody when O'Flair comes to the door, and Jody taps out a drum code for danger that Waldo taught her. Waldo knocks Glenson over, and while the doctor shoots it out with the police, PeeWee calls to say that Waldo is the band's new drummer. Jody announces her intention to marry Waldo, and they kiss.