Robert Siodmak's 1946 semi-noir The Dark Mirror is a tricky little mystery picture. It stars Olivia De Havilland (A Midsummer Night's Dream, The Snake Pit) as twin sisters accused of murder. When a respected doctor turns up dead, the girl working in the lobby of his office building is identified as the last person to have seen him alive. Only, what most people aren't aware is that the girl is really two girls--Ruth and Terry are twins, and they switch off from time to time. The fact that no one knows there are two of them means they can come and go as they please.
This throws a real wrench in the police investigation. Lt. Stevenson (Thomas Mitchell, It's a Wonderful Life) knows he's found his killer, but he can't figure out which twin did it. One was out at a concert in the park and has an airtight alibi, the other they claim to have been at home--but the girls won't incriminate themselves by saying which did what. If no witnesses can tell them apart, then only hard evidence or a confession will do. The case is officially closed for lack of proof.
Except Stevenson keeps it going on his own time, and he enlists the help of a psychologist who also works in the same building as Ruth/Terry. Dr. Elliott (Lew Ayers, the original Dr. Kildare) happens to be an expert on twins, and so this case offers him a new area of research. He will do a full case study of the sisters, and what he will find out should let him know which one of them is capable of murder. What he uncovers is a lot stickier than that, though: he falls in love with one of them, and he's convinced the other is clinically paranoid. The girls have a longstanding rivalry and have been known to compete for men. How your sympathies skew depends on which one you believe.
That's where The Dark Mirror gets tricky. Director Siodmak (Criss Cross), working from a story by Vladimir Pozner and a script by Nunnally Johnson (The Grapes of Wrath), isn't afraid to let his audience be confused. Even with De Havilland wearing necklaces and monograms that indicate which twin she is at any given time, it's often hard to tell them apart. Particularly in the early stages of the story when not even the personalities are clear; we get the sense that Terry is more exacting and tougher, and Ruth more meek, but it's hard to tell for sure which one was seeing the deceased man. The waters are as murky for us as they are for both the cop and the doc. De Havilland delivers a great performance, and she lets each twin's personality emerge slowly. The biggest difference is in how she carries herself and the openness with which she talks. Siodmak and cinematographer Milton Krasner (An Affair to Remember) pull off some remarkable effects for the time. More often than not, the effects shots featuring De Havilland acting on screen opposite herself are seamless. Literally. Only once or twice is the line between the two halves of the frame apparent. There is even a surprising shot where the actress embraces and comforts herself. The only time the filmmakers falter is when Terry is meant to be a reflection in the mirror while Ruth stands between her and the glass. The projection isn't entirely integrated, so the shot kind of fails. At the same time, given how the story hinges on a slip of the tongue regarding mirrors, I'd actually believe that the poor reflection is a deliberate symbol. Like Dr. Elliott's mumbo jumbo made real.
In the long run, there aren't a lot of surprises in The Dark Mirror. Siodmak sets up an effective red herring, but the deeper we get into the film, it grows increasingly apparent that the more predictable resolution is going to win out. That hardly matters, though, because the fine cast and Siodmak's excellent pacing keep the story moving and hold the audience's attention. The development of the romantic subplot takes center stage, and rather than wondering how the original victim ended up dead, our interests are diverted to wondering just how Dr. Elliott is going to see his way out of the predicament he's in. Much of the psychology is pretty dubious, and the film is romantic at heart, lacking the cynicism of a more traditional noir, but the guilty twin's last ditch efforts to get out of trouble makes for a manic, dramatic conclusion.