"Les Diaboliques" (Henri-Georg Clouzot, 1955) Oft-imitated classic thriller from the writers of "Vertigo" (Hitchcock tried to acquire the film-rights to this book, but lost out to Clouzot), and brought to low-key life by France's master film-maker. It's become the template for stories about duplicitous menage a trois' and twist endings, as well as a model of how to wring suspense out of an audience. And it's been remade a few times* (the last time with Sharon Stone and Isabelle Adjani and de-articled as simply "Diabolique."
Clouzot starts out slowly, bringing into detail the specifics of the unhappy/unholy union of cruel school-master Michel Delassalle (Paul Meurisse) and his weak (and weak-hearted wife) Christina Delassalle, (Véra Clouzot)and his mistress Nicole Horner, another school-teacher (played by Simone Signoret). The arrangement has been out in the open among the parties for awhile, and everyone is vaguely dissatisfied with it, so the only way to make everybody happy (well, almost everyone) is a simple little murder-plot. Three's a crowd. And as the school-master's a beast, he's the one to go.
Clouzot follows their plot with intricacy and humor, but with a disturbing eye for the horrific. His manipulation of dark-spaces (and subsequently, the audience) shows a master of technique, as well as creating amazing challenges for cinematographer Armand Thirard (whose work is simply amazing).
As with any film that sets a template, one might be able to know what will happen around the corner (it's the inevitability of the film's success and experience), but Clouzot's execution of the film is still a marvel to watch, and whether one suspects the outcome or not, it still provides considerable jolts to the system, and real moments of unease.
* I remember a TV movie with and Sam Waterston, Ira Levin's "Death-Trap" with Michael Caine and Christopher Reeve, and "Mortal Thoughts" starring Glenn Headley, Bruce Willis and Demi Moore. Hitchcock borrowed more than once from this film, just as one can say that Clouzot no doubt had the British director in mind, as well.