Les Diaboliques (1955), the French psychological horror thriller film directed by Henri Georges Clouzot, which shocked audiences in Europe and the USA before Alfred Hitchcock's Psycho, is created around a diabolical double-reverse plot that keeps the audience in suspense and guessing, till it reaches the unthinkable and thoroughly implausible final scene. The film, considered a classic of the genre and based on the novel Celle qui n’était plus (She Who Was No More) by Pierre Boileau and Thomas Narcejac, blends elements of thriller and horror, focusing on the fragile wife and the wilful mistress of her husband, the sadistic headmaster of a boys' boarding school, who hatch a daring plot to murder the man. With its unprecedented narrative turns and twists, together with awe-inspiring images, Les Diaboliques is a breathtaking benchmark in horror filmmaking, featuring outstanding performances by Simone Signoret, Véra Clouzot and Paul Meurisse.

The film is set in a decrepit French boarding school for boys, run by the tyrannical, cruel and sadist Michel Delassalle, played by Paul Meurisse, but owned by his teacher wife, the frail Christina, an emigrée from Venezuela, played by Véra Clouzot.

Christina inherited the school, bringing it into their marriage as part of her dowry, but Michel effectively takes all the decisions by himself, exhibiting his supremacy as the controlling man, taking the opportunity of Christian’s delicate constitution, the primary problem being a weak heart. Although their marriage has long been over because of Michel's rough and brutish behaviour towards her, Christina, as a devout Catholic, refuses to get a divorce. Michel has openly been having an affair with one of the teachers at the school, Nicole Horner, played by Simone Signoret, but even that illegitimate relationship is also on the rocks, as Michel mistreats and beats her. Instead of becoming antagonists, the two women have a somewhat close relationship, basically for their mutual hatred of Michel for his abusive treatment. Michael Delasalle, the sadist, is also a closefisted miser, who treats the boys cruelly, serves them rotten fish and slaps around his wife Christina, beats Nicole and also mocks Christina about her heart condition.

Unable to stand his mistreatment any longer, Nicole desperately hatches an elaborate plan to get rid of Michel forever and convinces Christina that their mutual problem can all be solved by the perfect execution of her plan. After initial hesitance, Christina ultimately consents to help Nicole and became a part of the conspiracy. It was decided that the elaborate plan to take place over a long weekend, when Nicole and Christina would drive down to Nicol’s house in Niort, a town several hundred kilometres away, for the weekend, while Michel is stuck at school.

Using a threatened divorce, Christina would then lure the brute to follow her to Niort in Nicole's apartment building and after that, the two women would drug and then drown him in Nicole's bathtub, drive back to the school with the body, disposing of it in the neglected and murky outdoor swimming pool of the school, which would eventually rise to the surface and thus be discovered. With the successful execution of the plan, it would appear that Michel committed suicide or died by accidentally drowning in the pool, while Nicole and Christina would have an alibi of being away for the weekend during the incident.

Despite a few modifications and a few hiccups, largely due to Christina’s nervousness, the two women could carry out the plan successfully, until the body fails to surface when the pool is drained. However, that was not all. The suit that Michel had been wearing at the time of drowning in the bathtub, was returned from the dry cleaners, along with a key to a room in a nearby hotel, found in a pocket.

When Christina visited the room, she was reported by the cleaning man that Michel had kept the room for a while, but rarely used it and stored nothing in the room. Meanwhile, in response to a report published in the Newspaper that police have found the corpse, Christina visits the morgue to identify the body, only to discover that it is not Michel’s body. However, there she meets Alfred Fichet, a retired senior policeman, now working as a private detective, who becomes involved in the case, much to the annoyance of Nicole.

Christina becomes shocked and sick, when she finds a student is punished by Michal, who is supposed to be dead, for breaking a Window. The impact of the shock made her so sick that she became unable to be photographed for the school photo, as the doctor advised her to take complete rest in bed to avoid any chance of fatality. Strangely, after the school photograph was taken, it seemed that Michel was very much in it, in the back, next to a window, just behind the students. Wildly overcome by fear, Christina tells everything to Alfred, who did not believe her, but thoroughly investigated the pool. That night, as Christina hears disturbing noises, she concludes that someone is in the school and wanders around the school, intending to find out the source. However, she runs back to her room, as she suddenly becomes aware that somebody is following her. But to her utter surprise, she finds Michel's corpse in her room, submerged in her bathtub and as the corpse of Michel rises from the tub, Christina died instantly of a massive heart attack.

In reality, Michel was not dead and everything was set by Michel and Nicole, with Michel acting as a vengeful ghost to scare Christina to death. While they were trying to escape, Alfred arrested both of them and the school was declared closed in the wake of the scandal. As the children and teachers were leaving, the boy who was punished for breaking a window, breaks another window with his slingshot and when asked how he got back his slingshot, the boy informs, Christina gave it to him.

Commonly compared to the films of Alfred Hitchcock, Les Diaboliques (1955) created a sensation upon its original release and was a success at the box office. It also won several awards, including New York Film Critics Circle Awards for Best Foreign Language Film, Edgar Allan Poe Award for Best Foreign Film and also earned Prix Louis Dellic, a French film award for Best Film. While Director Henri Georges Clouzot efficiently handled the suspense of the plot and increasing sense of dread, Simone Signoret, playing the role of Nicole Horner in her heyday as a fleshier, blowsier French version of Marilyn Monroe, made a dramatic contrast to the petite Christiana, played by Vera Clouzot, the director's wife. Only five years after its release, Véra Clouzot died of a heart attack, at the age of 46, mirroring her character in the film. Les Diaboliques, a compelling and grisly thriller, complete with an unforgettable twisted ending, is now considered a classic of the horror genre and film in general.