...even when some of those small things are not so small. 20 years ago Edith Phillips was going to marry wealthy Frank DeLorca. The wealth didn't really matter to Edith, but it mattered to her twin sister Margaret. It mattered so much that she slept with Frank and then announced she was pregnant, so Frank married Margaret out of obligation. There is still a shred of production code left, so I am explaining more than the film does. Edith has never forgiven Margaret for taking Frank away from her, but I'd say Frank does share some blame too since he must have slept with her.
So in modern day, which is where the film actually starts, Frank has died and Edith goes to Frank's funeral. This is where she finds out by the offhand remark of the DeLorca chauffeur that there never was a child. Margaret lied to both Frank and Edith to get the life of ease she wanted. In Edith's personal life she is about to lose the bar she runs because of back rent. She does good business but she is too generous for her own good. But that generosity does not extend to Margaret. Margaret, not knowing that Edith suspects the fake pregnancy business, goes to visit her in her apartment. After making double doggone sure that there never was a baby by getting Margaret to confess, Edith kills Margaret, changes outfits with her, dumps a suicide note in Margaret's lap that is supposedly written by Edith, and goes to take up Margaret's life as a DeLorca where she left off.
The thing is, what she really wanted that Margaret had - Frank - is dead, and the rest of it is rather empty without him. Plus there are any number of things to trip her up, starting with the fact that she knows none of the servants, does not know her way around the DeLorca mansion, and does not know Margaret's routine or friends. On top of this Margaret had a boyfriend on the side who is not exactly a catch - an obvious fortune hunter played by a rather fat and flabby Peter Lawford. And he is blackmailing "Margaret" and for good reason. Complications, some very ironic, ensue.
Davis still has it as she convincingly plays the working class Edith, the pampered Margaret, and Edith masquerading as Margaret quite convincingly. Karl Malden is quite touching as Edith's cop boyfriend. He has two possible endings to Edith's story, neither which is flattering, and he doesn't know which to believe. The jazz band playing over Edith's killing of her sister and covering it up adds to the tension of the scene.
I'd recommend this one.
