At a concert performed by the renowned pianist Anthony Lawrence, Dr. Igor Markoff, a mad scientist listening in the audience, becomes bewitched by Lawrence's daughter Patricia. After Lawrence finishes playing, Markoff hurries backstage to meet Patricia and explains that she is the living image of his late wife Leonora. When Patricia, who is engaged to her father's manager, Bob Blake, shows little interest in the doctor's romantic overtures, Markoff bribes the theater doorman for information about the Lawrence family. Markoff's interest in Patricia evokes jealousy in his faithful assistant Maxine, who is in love with the doctor herself. When Markoff begins to send daily bouquets of flowers to Pat, she beseeches her father to ask the doctor to cease his unwelcome attentions. Visiting Markoff in his laboratory, Lawrence insists that he stop annoying Pat. When Markoff replies that he intends to marry Pat, Lawrence threatens to call the police, and the doctor strikes him and knocks him out. While Lawrence is unconscious, Markoff injects him with a serum that causes acromegaly, a disease of the pituitary gland that over time hideously enlarges the patient's extremities. Markoff then calls the unsuspecting Pat to drive her father home, and after they leave, Maxine warns the doctor that she knows he killed the real Markoff and assumed his identity. The callous Markoff then divulges that he killed the scientist because the man was in love with Leonora. To punish his wife, Markoff injected her with the acromegaly serum, and when she could no longer tolerate her hideous appearance, she killed herself. Some time later, Pat becomes concerned by her father's swollen extremities and urges him to see their physician, Dr. Adams, who diagnoses Lawrence's disease as acromegaly and refers him to Dr. Markoff, an expert in the field. Weeks later, Bob visits Pat after an extended absence and discovers that Lawrence has locked himself in his room. Later that night, Bob and Pat surprise Lawrence in his darkened music room, and Pat, horrified by her father's appearance, screams and faints. As Bob carries Pat to her room, Lawrence drives to Markoff's lab and accuses the doctor of infecting him, Just as Lawrence is about to kill Markoff in vengeance, the doctor's orderly enters the room and restrains him. After Markoff sedates Lawrence, Maxine, who has overheard their entire conversation on the intercom, threatens to expose him. That night, after Maxine goes to bed, Markoff releases his vicious ape and guides the animal to Maxine's room, intending to kill her. Maxine's loyal dog Ace comes to her rescue, however, and the next morning the doctor, surprised that his assistant is still alive, sends her to the pharmacy on an errand. Upon discovering that her father is missing, Pat calls Markoff, who informs her that Lawrence is there for observation. Hurrying to Markoff's lab, Pat asks her butler to tell Bob when he arrives at the house. Meanwhile, Markoff offers to cure Lawrence in exchange for Pat's hand in marriage, but Lawrence refuses. When Pat arrives, Markoff takes her to see her father, who is shackled to his bed. Soon after, Maxine returns home and when she tries to come to Pat's assistance, she is restrained by Markoff's orderly. Just then, Bob, who has followed Pat, enters the house, knocks out the orderly and frees Maxine. When Markoff tells Pat that he will cure her father if she agrees to marry him, Lawrence breaks his bonds and attacks the doctor. Pat runs into the hallway and is met by Maxine and Bob. Hearing a gunshot, they rush back into Lawrence's room and discover the doctor, lying dead on the floor. When Pat bemoans that without the doctor's help, her father will suffer a hideous death, Maxine declares that she knows the secret of Markoff's serum and can cure him. Some time later, a healthy Lawrence resumes his concert tour.