During her engagement party, Joan Ellis (Phyllis Thaxter) hears the voice of another personality called Karen within her. Karen forces Joan to leave her fiancé, Bob Arnold (Henry H. Daniels Jr.), and move to New York. There she meets lawyer Eric Russell (Stephen McNally), whom she falls for, but again Karen compels her to flee. When she returns to Bob, Karen takes control and murders him. Joan's fate then hinges upon Dr. Bergson (Edmund Gwenn) being able to uncover her illness.