When a man is discover stabbed in the back witnesses point out the woman they saw at the scene of the crime, the demur Ruth Collins (Olivia de Havilland) but their eye-witness account goes bad when they see Terry Collins (Olivia de Havilland) the twin sister. During their psychically and physically connected lives the two have actually pretended to be each other for many reasons like subbing for one another at work and other activities. In this case it seems that one cant believe that the other could do anything so horrible. On the case is Lieutenant Stevenson (Thomas Mitchell) but when he realizes that he cant bring in the killer without the aid of the twin, he turns to Dr. Scott Elliott (Lew Ayres) to do a psychological profile of the women to see if the real murderess can be spotted. When Dr. Elliott realizes that one twin is an insane killer the situation turns deadly.