Doctor Peralta has been found murdered, and Lieutenant Stevenson (Thomas Mitchell) is on the case to work out who the culprit is, but he soon hits a snag. There was a woman seen with the victim just before he died, and the witnesses were able to offer an excellent description, with the suspect being Terry Collins (Olivia de Havilland) who worked at the newsagents where Peralta's offices were. When Stevenson confronts her, she faints dead away, but recovers enough to go home where he follows, keen to interview her. This is where the snag enters into the investigation...
Which is Terry has a twin, Ruth, also played by de Havilland, and neither of them will admit which one of them was with the victim that fateful night. So begins one of the most contrived film noirs of the forties, which featured a depiction of both police work and psychiatry that were utterly laughable - but that was why it was so enjoyable to watch. The Dark Mirror was directed by a past master of the noir technique, Robert Siodmak, and the script was penned by producer Nunnally Johnson, one of the most caustic wits in Hollywood and also responsible for some of its biggest hits of his day, making him the highest paid screenwriter at the time.
Accompany that with a performance, two in fact, by de Havilland where she got to show off her range as both the good twin and the evil twin, and you had purest hokum which carried with it an old time movie entertainment value no matter how ridiculous it came across, both then and now. Olivia had a field day as one of her usual nice girls contrasting with a rare chance to play the bad girl, but deftly keeping us guessing as to which was which - for a while at least, for when we had to distinguish their true identities she was just as effective at putting that across as well. Trick photography was implemented to put both Collins in the same frame, and a double filmed from behind but dressed identically was useful for sustaining the illusion, so much so that you didn't notice it once you were caught up in the plot.
This being the forties and a thriller, everyone was obviously eager to put the then-fashionable psychoanalysis to good use, so to work out who was the actual murderer in this case - for some movie contrivance reason neither is allowed to be arrested, leaving any resolution up in the air - Stevenson encourages a psychiatrist to become involved. He is Dr Scott Elliott, played by a famous screen medical man Lew Ayres who had been such a success as the noble Dr Kildare in the film series. Here he was evidently none too bothered about ethics as once he gets to know Ruth he fancies a romantic entanglement, though you could excuse this because he hadn't officially adopted her as a patient, and besides he was also examining Terry.
One of them is far more devious than the other, even to the extent of trying to drive her sister mad by pretending she's suffering hallucinations and therefore having her believe she could have carried out the killing when she was actually innocent. In the psychiatrist's chair, a series of clichés see to it that both ladies are put through the Rorshach test of ink blots, a word association game, and most egregiously of all a lie detector - even back then that last was open to so much doubt that it was next to useless. Not to Hollywood, of course, and the way we see the needle jump when the crazy sister gets to talking makes up our minds that she's the one we need to watch out for. Naturally, the more straight-faced in its absurdity this gets, the more fun it is, and while Ayres is his dependable self even with his character's lapses - he looks like a lounge lizard here - and Mitchell is amusingly baffled as to how he can solve his case, it was Olivia's movie, the benchmark for all twin performances in thrillers. Music by Dimitri Tiomkin.