Dr. Frank Peralta is stabbed to death in his apartment one night. The detective on the case, Lt. Stevenson (Mitchell), quickly finds multiple witnesses putting Peralta's lover, Terry Collins (de Havilland), at the scene. However, when Stevenson finds Terry and questions her, she has an iron-clad alibi with multiple witnesses. It is revealed that Terry has an identical twin sister, Ruth (de Havilland), and the pair share the same job and routinely switch places for their own benefit. Stevenson and the district attorney are unable to prosecute, since the twins refuse to confirm which one of them has the alibi.
Unable to accept the "perfect crime", Lt. Stevenson asks Dr. Scott Elliot (Ayres) for help. Scott is an expert on twin study, and has been routinely encountering the Collins twins at their shared place of work, but does not know which one is which. As a front, Scott asks Terry and Ruth if he can study both of them individually as part of his research. The twins accept, though Ruth is worried that Scott might find out that Terry was at Peralta's apartment the night of the murder. However, Terry is attracted to Scott and insists that they can keep the secret for the sake of seeing him. She also comforts Ruth, reminding her that she was only at Peralta's apartment but didn't kill him.
From Scott's psychological tests and by spending time with them, he discovers that Ruth is kind and loving, while Terry is highly intelligent, insane, and has been manipulating Ruth almost their entire lives. Terry is jealous that people keep preferring Ruth over her, and is again enraged when Scott falls in love with Ruth instead of her. Terry starts methodically gaslighting Ruth, making her believe that she's hallucinating and going insane, in the hopes of pushing her to suicide.
Scott reports his findings to Stevenson, who advises him to warn Ruth immediately. That night, Scott arranges to meet with Ruth at his apartment, but Terry intercepts the message. Terry leaves Ruth alone at their apartment, and sets a music box in a hidden place to further drive Ruth to madness. Terry goes to meet Scott, but he's aware of who she really is. Scott explains everything he's learned about the twins' relationship and Terry's intense rivalry with her innocent sister. Scott also believes that Peralta, who didn't know they were twins, wooed Terry but was really in love with Ruth, and Terry killed him for it. Just as Terry is considering stabbing Scott, he receives a phone call from Stevenson, who is at the twins' apartment, having visited her on a hunch and found Ruth dead.
Scott and Terry go to the sisters' apartment, where Terry "confesses" to Stevenson that her "sister" killed Peralta and committed suicide out of guilt. Terry confirms all of Scott's psychological test results, but she herself claims to be Ruth, and says that she's relieved that "Terry" is dead. Just then Ruth enters the room, alive and well, which causes Terry to throw her glass to a mirror in anger. Stevenson did visit Ruth on a hunch but only found her in distress, not dead; he then faked the phone call in order to trap Terry, who is arrested.Olivia de Havilland as twins Terry and Ruth Collins
Lew Ayres as Dr. Scott Elliott
Thomas Mitchell as Lt. Stevenson
Richard Long as Rusty
Charles Evans as Dist. Atty. Girard
Garry Owen as Franklin (as Gary Owen)
Lela Bliss as Mrs. Didriksen
Lester Allen as George Benson
When first released the staff at Variety magazine gave the film a mixed review, writing, "The Dark Mirror runs the full gamut of themes currently in vogue at the box office - from psychiatry to romance back again to the double identity gimmick and murder mystery. But, despite the individually potent ingredients, somehow the composite doesn't quite come off...Lew Ayres is cast in his familiar role as a medico - a specialist on identical twins. Slightly older looking and sporting a mustache, Ayres still retains much of his appealing boyish sincerity. But in the romantic clinches, Ayres is stiff and slightly embarrassed looking. Copping thespic honors, despite a relatively light part, Thomas Mitchell plays the baffled dick with a wry wit and assured bearing that carries belief."
Bosley Crowther, film critic for The New York Times, also was critical, writing, "The Dark Mirror, like so many of its ilk, suffers from its author's lack of ingenuity to resolve his puzzle in a satisfying manner. As in his earlier and superior mystery, The Woman in the Window, Mr. Johnson solves the problem with a bit of trickery which is no credit to his craftsmanship. Still, one must hand it to Mr. Johnson for keeping his audience guessing, if not always entertained."
Film critic Dennis Schwartz liked the film and wrote, "Siodmak deserves high praise for keeping the melodrama agreeable and intelligent when dealing with both the doppelgänger effect and the sibling rivalry; he does it better than the way most films have covered those subjects, showing that identical twins might look the same but have different psychological attitudes."
Dark Mirror was remade as a TV film in 1984, with Jane Seymour as the twins, Stephen Collins as the Psychiatrist and Vincent Gardenia as the cop.