Elizabeth (Eleanor Parker) has recurring headaches and is plagued with insomnia. She is receiving letters from a woman called Lizzie, but Elizabeth can't remember knowing anyone named Lizzie. When Elizabeth is under hypnosis, her psychiatrist, Dr. Wright (Richard Boone), discovers Elizabeth has three personalities: The shy Elizabeth, the Dr. Jekyll and Mr. Hyde-like Lizzie, and the kind, well-adjusted Beth, the woman she always should have been. It is up to Dr. Wright to help Elizabeth to become Beth completely.