The Dark Mirror is a 1946 film directed by Robert Siodmak.
The film opens with one Dr. Frank Peralta lying on the floor of his apartment, stabbed to death. Investigating officer Lt. Stevenson (Thomas Mitchell) soon finds multiple witnesses placing Peralta's girlfriend, Terry Collins (Olivia de Havilland) at the scene. Stevenson is confident that he has his woman—until Terry reveals that she has an ironclad alibi, as multiple witnesses saw her elsewhere that night.
Only it isn't Terry! It's Terry's identical twin sister and roommate Ruth (also De Havilland, of course) who was seen by multiple witnesses at a concert and walking in the park. Or was it Terry? All the sisters will tell Stevenson is that one of them has an alibi, and the other was home alone. They refuse to say which one is which. A frustrated Stevenson realizes that without any way to show which sister was the one seen at Dr. Peralta's apartment, he can't arrest either of them.
With no other option, Stevenson approaches Dr. Scott Elliot (Lew Ayres), another doctor in the same office building where Peralta practiced and where the sisters run a concession stand. Elliot is a psychiatrist who just happens to be conducting research on twins. Elliot recruits the Collins sisters for one of his twin studies. While finding out some dark things about Terry, Elliot starts to develop feelings for Ruth. Terry does not like this one bit.
One of only two films where Olivia de Havilland played a villain, Hush...Hush, Sweet Charlotte being the other. The first film Lew Ayres made in four years, after he was fired from the Dr. Kildare series for supposedly refusing to serve in World War II—in fact Ayres had volunteered to serve in a non-combat role, and did serve as a front-line medic in the Philippines.
- Cigarette of Anxiety: Terry, who is marked out as being the Evil Twin by being a smoker, really smokes up a storm after she sees Ruth and Elliot kissing on the front steps.
- Contrived Coincidence: Sure does work out well that the guy with an office right next to the twins' concession stand is a specialist on identical twins.
- Coordinated Clothes: The twins often wear matching outfits.
- Evil Twin: Terry, it turns out, is a sociopathic murderer, driven to fits of violence by her pathological jealousy of Ruth.
- Fainting: Ruth faints when Stevenson tells her that Dr. Peralta is dead.
- Gaslighting: Terry starts doing this to Ruth after becoming jealous of her and also becoming suspicious that Ruth will rat on her. She uses such tricks as turning the lights on quickly in the middle of the night and telling her now-awake sister she must be hallucinating, and hiding a music box in the house and leaving it on.
- Good Smoking, Evil Smoking: How do we know that Terry is the Evil Twin? Because she smokes, of course.
- Have You Told Anyone Else?: Terry says "Who else have you told this to?" when Elliot reveals that he has figured out that she is insane and she killed Dr. Peralta. Soon after she is reaching for scissors to stab him In the Back with, but he turns just before she picks them up.
- Identical Twin ID Tag: While the sisters often wear Coordinated Clothes, the clothes usually have a T or R monogram. And when they're not wearing the monogrammed outfits they're wearing necklaces that actually spell out "TERRY" or "RUTH".
- If I Can't Have You...: Why Terry killed Dr. Peralta and why she almost kills Elliot. She fell in love with both of them, only to figure out that each of them preferred Ruth.
- Inkblot Test: One of the tests that Dr. Elliot runs on the twins. Terry's responses to the ink blots are just a little bit weirder.
- Lie Detector: Another test that Dr. Elliot runs on the twins. He observes that Terry's lie detector graph spikes crazily whenever he asks her a question about Ruth.
- Plot Hole: So, Dr. Peralta was courting Terry while actually preferring Ruth, only he didn't know they were twins...so what name did he call them? If he called the woman behind the counter "Terry" then at some point Ruth would have surely corrected him, and if he called her "Ruth" then Terry would have known right off the bat that Peralta was confused.
- Sibling Triangle: Dr. Peralta was in one without knowing it. The sisters took turns manning the counter, and while Dr. Peralta wound up taking Terry out, he actually preferred Ruth. When Terry found this out, she killed him. Later, Terry takes a fancy to Dr. Elliot, only to be sent into another jealous fury when she sees him kissing Ruth.
- Smart People Play Chess: Just to make sure the audience knows Dr. Elliot is smart, he's shown fiddling with a chess board at home.
- The Sociopath: Dr. Elliot calls Terry a "paranoiac", but he does describe her as having no conscience and no sense of right or wrong.
- Twin Switch: The twins were in the habit of alternating shifts at the concession stand without anyone else being the wiser. At the end, after being told that Ruth has killed herself, Terry starts insisting that she is Ruth.
- Word Association Test: Another of the tests that Dr. Peralta runs on the twins. It's a big dramatic moment when each of them associates "mirror" with "death".