This is one of those films that you really have to pay close attention to - otherwise you can miss the nuances and menace created by the super performance from Olivia de Havilland - who plays two twins - in this whodunit with lots of twists. When "Dr. Petralta" is found stabbed through his heart - witnesses swear to policeman Thomas Mitchell ("Lt. Stevenson") that they saw "Ruth" leave his apartment moments after the grizzly deed was supposed to have been committed. Upon further investigation, however, he discovers that "Ruth" has a cast iron alibi! She also has a twin sister "Terry", and so proving who did what to whom - they both, naturally, deny any involvement - is nigh on impossible. That is, until scientist "Dr. Elliott" (Lew Ayes) who specialises in the behaviour of twins starts to study the pair and slowly begins to discover their secrets. I struggle to think of a better role for the star, she is clever and evil, calculating and cunning in her portrayal and whilst there is little doubt that one of them "did it" - it takes it's time to prove which one did - and I found the trails of breadcrumbs left for the audience made for a compelling thriller as it becomes quite clear that one sister is more than willing to throw the other under the bus!