At her engagement party, Joan Ellis, your typical girl next door, acknowledges that the strange voice that she's heard all her life is coming from within herself. Joan's alter ego, a conniving and murderous woman named Karen, is becoming stronger and stronger and is threatening to take over her life. Joan negotiates with Karen. Although they do come to an agreement, Karen reneges on that agreement when the situation suits her. Karen ultimately wants the men in Joan's life, first her fiancé, Bob Arnold, then Eric Russell, a lawyer Joan meets in New York City. Joan does whatever she needs to to get rid of Karen, even if it a threat to her own life. Without knowing for certain what is wrong with Joan, those close to her, including Eric and Dr. Bergson, do whatever they can to save her.