When a prominent physician is found stabbed to death, police detective Stevenson's investigation leads him directly to magazine store clerk Terry Collins, who works in the doctor's office building and is identified by co-workers as the murdered man's girlfriend. Stevenson is puzzled by the conflicting descriptions of Terry he hears from people with whom she works, even that of another doctor, Scott Elliott. When Stevenson goes to Terry's apartment, she introduces him to her identical twin sister Ruth, and he understands the witness' discrepancies. The sisters admit that they often fill in for each other at the magazine stand, but when neither will confirm her whereabouts the night before, Stevenson arrests both. During a police lineup, however, none of the eyewitnesses can tell the sisters apart, so Stevenson is forced to release them. Ruth is nervous and upset by the incident, while Terry is angry. Stevenson seeks assistance from Scott, who was consulting with the murdered doctor on the topic of split personalities. Stevenson is sure one of the twins committed the murder and wants Scott's help in proving it. Scott explains it is possible to do character analysis tests to indicate whether either sister has the proper psychological makeup to commit murder. Claiming that he is conducting the research on sibling personalities, Scott then asks Ruth and Terry if they will participate in a study. Ruth is wary of the procedure and refuses, but Terry accepts and soon reveals that she was the murdered doctor's girl friend. She also admits Ruth was willing to lie to protect her to keep them together, but denies having anything to do with the killing. At home Terry talks Ruth into enrolling in Scott's study and eventually Ruth agrees. Scott gives both the Rorschach ink blot test, in which each sister is asked to describe what she sees in a variety of ink blots. Later, Scott conducts a word association test and Ruth's linking the word "mirror" with "death" upsets Terry. Terry questions whether Ruth doubts her in any way, but Ruth denies it, although she remains uneasy about Scott's questioning and cannot sleep, a long-standing condition for which she regularly takes sleeping pills. Scott begins seeing Ruth casually outside of the office and comments about the deep-seated competitiveness between the two sisters. Mildly surprised, Ruth admits the only real difference between her and her sister is that Terry has never liked any of Ruth's boyfriends. At the end of one of their walks to the Collins' apartment, Scott kisses Ruth, which Terry observes from the window. Later, Terry asks Ruth if she has been feeling ill as, despite the sleeping pills, she is agitated each night, crying out. Ruth has no memory of any disturbing dreams and is upset by Terry's suggestion that she is worried about possible insanity, which is rumored to occur in twins. A little later, Scott conducts a final exam on each sister, a polygraph test. During Terry's test, Scott asks about Ruth's early boyfriends, and Terry's embellishments of Ruth's story cause the recorder to vacillate wildly, indicating she is lying. That night Ruth is startled out of her sleep and panicked by bright flashes, unaware that Terry intentionally has caused them. The next day, Scott informs Stevenson that his tests clearly indicate Ruth is incapable of committing murder, but that Terry is paranoic. Stevenson believes Ruth may be in some danger from her sister and asks Scott to tell Ruth the results immediately. Scott telephones the Collins', reaching Terry, who pretends to be her sister and promises to meet Scott that evening at his apartment. As Scott hangs up the phone, Ruth walks into his office, somewhat unsettled. Scott takes her out and she asks him if it is possible she is suffering a mental breakdown. He assures her she is mentally sound, but does not give her the examination results. Later after Ruth returns home, Scott calls Stevenson to tell him he is meeting Terry, who is pretending to be Ruth. When Terry arrives, masquerading as Ruth, Scott proceeds to give her the true test results, stating that Terry is never successful in romantic situations and that some childhood incident has provoked years of jealous resentment and hostility in her. After Terry grows furious at this diagnosis, Scott acknowledges her true identity, then accuses her of murdering the doctor. Terry responds by declaring that Ruth has suffered a nervous breakdown and committed the murder. Just then, Stevenson telephones from the sisters' apartment with disturbing news about Ruth and asks the two to come to the Collins' apartment. Upon their arrival, Stevenson announces that Ruth has committed suicide. Terry remains calm, claiming her sister was responsible for the murder and plagued by it. She then asserts that she is Ruth and that Terry killed the doctor out of jealousy, as he preferred Ruth. At the height of this confession, Ruth walks out of the bedroom and, in an hysterical fury, Terry throws an object at her sister's reflection in the mirror before collapsing. After Terry has been arrested, Stevenson explains to Ruth and Scott that he felt a trauma was necessary to force Terry's confession and that Scott was in as much danger from Terry as Ruth. When Stevenson departs, Scott comforts Ruth.