Who does the deceased's lover look like?
Dr. Frank Peralta is stabbed to death in his apartment one night. The detective on the case, Lt. Stevenson, quickly finds multiple witnesses putting Peralta's lover, Terry Collins, at the scene. However, when Stevenson finds Terry and questions her, she has an iron-clad alibi with multiple witnesses. It is revealed that Terry has an identical twin sister, Ruth, and the pair share the same job and routinely switch places for their own benefit. Stevenson and the district attorney are unable to prosecute, since the twins refuse to confirm which one of them has the alibi.
Unable to accept the "perfect crime", Lt. Stevenson asks Dr. Scott Elliot for help. Scott is an expert on twin study, and has been routinely encountering the Collins twins at their shared place of work, but does not know which one is which. As a front, Scott asks Terry and Ruth if he can study both of them individually as part of his research. The twins accept, though Ruth is worried that Scott might find out that Terry was at Peralta's apartment the night of the murder. However, Terry is attracted to Scott and insists that they can keep the secret for the sake of seeing him. She also comforts Ruth, reminding her that she was only at Peralta's apartment but didn't kill him.
From Scott's psychological tests and by spending time with them, he discovers that Ruth is kind and loving, while Terry is highly intelligent, insane, and has been manipulating Ruth almost their entire lives. Terry is jealous that people keep preferring Ruth over her, and is again enraged when Scott falls in love with Ruth instead of her. Terry starts methodically gaslighting Ruth, making her believe that she's hallucinating and going insane, in the hopes of pushing her to suicide.
Ruth