Diabolique

Diabolique is a movie starring Sharon Stone, Isabelle Adjani, and Chazz Palminteri. The wife and mistress of the sadistic dean of an exclusive prep school conspire to murder him.
The wife and mistress of a cruel school master collaborate in a carefully planned and executed scheme to murder him. The plan goes well until the body, which has been strategically dumped, disappears. The psychological strain starts to weigh on the two women when a retired police investigator begins looking into the man's disappearance on a whim.