In a dilapidated French boarding school, Christina Delassalle, the frail wife of the cruel headmaster Michel Delassalle, teams up with his mistress, Nicole Horner, to end his reign of terror. Michel's sadistic nature pushes both women to the brink, leading them to concoct a sinister plan to murder him and dispose of his body. They lure Michel to Nicole's apartment, sedate him, and drown him in a bathtub, subsequently transporting his body back to the school and dumping it in the murky swimming pool, expecting it to be discovered in due course.

However, things take a disturbing turn when the pool is drained, revealing no trace of Michel's corpse. As Christina grapples with the growing tension and guilt, bizarre occurrences unfold within the school. Michel's ghostly presence seems to linger, unsettling the already frightened Christina. The inexplicable events lead her to question her sanity and fear that something or someone knows about their crime. The carefully laid plan begins to unravel as the two women find themselves caught in a web of deceit and horror, with the chilling realization that the nightmare is far from over.