The wife and 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' (and the audience) up the wall with almost unbearable suspense...
In a French provincial town, Michel Delassalle, a sadistic headmaster of a school belonging to his wife Christina, a fragile young woman with a weak heart, carries on an affair with Nicole Horner, a strong, forceful teacher who has been his mistress from the day she arrived. He has, however, treated her as badly as his wife, and the two women have been driven into an alliance against him. Together they work out an elaborate plan to rid themselves of their common tormentor. Luring him away from the school to Nocole's cheap lodging house, they induce him to drink some doctored whiskey - and drown him in a bath. The body is later wrapped in a nylon tablecloth, packed into a laundry basket, taken back to the school, and at dark tipped into the grimy water of the school swimming pool. When, shortly after, the pool is drained, watched in anguished expectation from a window by the women, no corpse is there. Soon other mysterious events begin to occur...
Christina Delasalle suffers greatly at the hands of her brutish husband Michel. She inherited the boys boarding school they run but it's clearly Michel who is in charge. With Nicole Horner - one of the teachers and Michel's former lover - they decide to kill him. They lure him to Nicole's house where he is drugged and drowned in the bath. They then dispose of his body in the school murky and algae-filled swimming pool. After several days, his body fails to float up to the surface and when the pool is drained, his body is nowhere to be found. Christina, who has a serious condition, is terrified especially after one of the students says he saw him. By chance, she meets a retired police inspector who decides to look into the case.
Married Michel and Christina Delassalle are the headmaster and headmistress respectively of Institution Delassalle, a second rate boys boarding school in St. Cloud, outside of Paris. Although Christina officially owns it - she bringing it into their marriage as part of her dowry - Michel effectively makes all the decisions, partly because he wants to show everyone involved that he is in control as the man, and as Christina is a frail woman with a delicate constitution, the primary problem being a weak heart. Their marriage has long been over because of Michel's brutish behavior toward her, although Christina, a devout Catholic, refuses to get a divorce. Michel has openly been having an affair with Nicole Horner, one of the teachers at the school. But even now their relationship is on the rocks as Michel beats her. Nicole convinces Christina that their collective problems can all be solved if they kill Michel. Nicole hatches an elaborate plan to take place over a long weekend: she and Christina will drive down to her house in Niort for the weekend, while Michel is stuck at the school; Christina will make up some ruse for Michel to come to Niort; they will drug and then drown him in Nicole's bathtub; they will then transport the body back to the school before the end of the weekend, dumping the body into the school's murky outdoor pool; the dead body will eventually rise to the surface and thus be discovered. With the successful execution of the plan, Nicole and Christina will have an alibi of being away for the weekend when he would have died by accidentally drowning in the pool. With a few required modifications and a few hiccups along the way, the latter largely due to Christina often losing her nerve, they are able to carry out the plan successfully... or so they believe. However, the body never materializes in the pool, even after the pool is emptied on a pretense. Other strange things happen with regard to a missing Michel which belie what Nicole and Christina believe to be reality. Did someone discover what they did, take the body and will eventually try to blackmail them? Is Michel still alive and just playing with them? Are their minds playing tricks on them? Or is there another reason they just have not thought about as a possibility?
The wife and mistress of a cruel headmaster conspire to kill him; after the murder is committed, his corpse disappears, and strange events begin to plague the two women.
- 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 mutual 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 the 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.
When Christina and Alfred come back, a boy is punished for breaking a window; the boy says Michel punished him. After hearing this Christina becomes very sick. She is unable to be photographed for the school photo; however, it seems that Michel is in it, in the back next to a window. Nicole becomes worried and leaves the school. Christina, overcome by fear, tells Alfred everything. He does not believe her, but he investigates the pool. Christina hears some noises and wanders the school. She concludes that someone is in the school and she runs back to her room. She finds Michel's corpse in the bathtub. Michel rises from the tub, and Christina has a heart attack and dies.
Michel and Nicole have set up Christina from the beginning. Michel is not dead, but acting dead to scare Christina to death. But as soon as Nicole and Michel escape Alfred is there to arrest them.
As the movie ends, the same boy who had earlier broken a window breaks another. When asked how he got his slingshot back, the boy says that Christina gave it back to him. A final title screen tells the audience not to reveal the ending to others.