It requires real focus to fully appreciate this movie; overlooking subtle details risks missing the unsettling atmosphere and Olivia de Havilland’s remarkable dual performance. She portrays twins in this mystery, full of unexpected turns. After "Dr. Petralta" is found fatally wounded, witnesses tell policeman Thomas Mitchell ("Lt. Stevenson") that they observed "Ruth" leaving his apartment shortly after the crime supposedly occurred. However, a deeper look reveals “Ruth” has an airtight defense! She also happens to have an identical twin sister, "Terry," making it extremely difficult to determine responsibility – both sisters maintain their innocence. The situation doesn't resolve itself until scientist "Dr. Elliott" (Lew Ayres), who studies twins, begins examining them and gradually uncovers their hidden truths. I can’t imagine anyone more suited to the part; her depiction is intelligent and malicious, displaying a calculating nature. While it seems certain that one of them committed the act, the film deliberately prolongs the revelation, and I found the clues provided for the audience created an engaging suspense as it becomes apparent that one sister is prepared to betray the other.

Source on TMDB