Diaboliques (1955) (1955)

Christina Delasalle, a fragile young wife with a weak heart, and Nicole Horner, a strong, forceful mistress of a sadistic boarding school headmaster plot to kill him. They drown him in the bathtub and dump the body in the school';;s filthy swimming pool... but when the pool is drained, the body has disappeared - and subsequent reported sightings of the headmaster slowly drive his ';;killers';; up the wall with almost unbearable suspense...