Les Diaboliques Movie Wiki

The story takes place in a second-rate boarding school run by the tyrannical and mean Michel Delassalle (Meurisse). The school, though, is owned by Delassalle's teacher wife, the frail Christina (Clouzot), and Delassalle flaunts his relationship with Nicole Horner (Signoret), a teacher at the school. Rather than antagonism, the two women are shown to have a somewhat close relationship, primarily based on their apparent common hatred of Michel, who is physically and emotionally abusive to both.
Unable to stand his mistreatment any longer, Nicole devises a plan. Though hesitant at first, Christina ultimately consents to help Nicole. Using a threatened divorce to lure Michel to Nicole's apartment building in a remote village several hundred kilometers away, Christina sedates him. The two women then drown him in a bathtub and dump his body in the school's neglected swimming pool. When his corpse floats to the surface, they think it will appear to have been an accident. Almost everything goes according to their plans until the body fails to surface, and Michel's corpse is nowhere to be found when the pool is drained.
Nicole sees in the paper that the police found a corpse. Christina goes to the morgue and learns it is not Michel's body. There she meets Alfred Fichet (Vanel), a retired private detective. He gets involved in the case, much to Nicole's chagrin.