Les Diaboliques (aka Diabolique) (1955)

Set at a lower tier boarding school for boys in Paris, the mistress (Simone Signoret) and the wife (Vera Clouzot) of a sadistic womanizer (Paul Meurisse) plot to murder him. Based on the novel CELLE QUI N'ETAIT PLUS by Pierre Boileau and Thomas Narcejac and directed by Henri Georges Clouzot (WAGES OF FEAR). This cleverly crafted thriller is one of those influential films that have been so endlessly imitated since its debut that audiences new to the film may not get why it is considered a classic in the genre but its impact on generations of film and film makers cannot be ignored. It was one of the first films with a "twist" ending (unless you count those "it was all a dream" movies from the 1940s). Clouzot imbues the film with a strained atmosphere that only increases as the plot progresses. As the mistress and abusive husband, Signoret and Meurisse are perfect and if the film has a weakness, it's the director's wife who can't quite hold her own with her acting partners. The film was a sensation with arthouse audiences and critics in the U.S. but the British critics were rather prissy toward it. With Charles Vanel, Michel Serrault and Pierre Larquey.