Based on a novel by Pierre Boileau and Thomas Narcejac, the wife and mistress of a selfish school principal carry out their plan to murder him, but the result is rather startling.
In Paris at the Delasalle Boarding School a bell calls students to class. Nicole Horner (Simone Signoret) has a bruised eye, and she tells Christina Delasalle (Véra Clouzot) that it was Michel when he came back at 3 a.m. Christina says that he made her happy at the beginning. Nicole says he wanted her money. Michel Delasalle (Paul Meurisse) sees them and asks Nicole for a kiss. He realizes she is still angry, and then he kisses Christina. Nicole says that Christina is delicate, but Michel does not agree she is anymore. He tells them to get out and pushes them. Christina and Michel disagree about disciplining a boy.
Christina and Nicole sit apart from others on the playground. Christina says she could not get a divorce, and she no longer loves Michel. She says divorce is a deadly sin, but Nicole asks her if her carrying around a knife was a deadly sin. Christina says she was jealous. She is reluctant, but Nicole says they have an opportunity in the next three days. Nicole does not believe in hell, but Christina says she does.
Old fish is served at the school cafeteria, and Michel refuses to eat it. He tells Christina to eat. Michel tells the children to be quiet and threatens to keep them over the weekend. He tells Christina to swallow because the children are looking. The children begin chanting, and Michel tells two teachers to throw them out. Michel says he buys better fish but at a discount. Christina says he is using her money to pay them, and she is upset. Nicole tries to calm her down. Christina says she would like to die, and Michel encourages her. Nicole says she is leaving in the morning, and he wishes her a nice vacation.
While Michel is asleep, Christina leaves with a suitcase. She finds Nicole in the garage. At breakfast Michel learns that Christina and Nicole left. They both wish Michel were dead, but Christina is not sure. Nicole says that Michel told her that Christina has a bad heart and would not last long. They drive to Niort. They carry in Nicole’s large trunk, and Mme. Herboux (Therese Dorny) is introduced to Christina. Nicole asks for her rent. Nicole places a call for Christina, who tells Michel that she wants a divorce and her money in the school back. Michel objects and says he is coming on the train.
Nicole says she bought a nylon tablecloth that will fit him. Nicole opens some expensive wine and tries it. Christina watches her put a few drops in the bottle and asks if it leaves traces. Nicole says it will make him sleep. They hear his train. Nicole starts to fill the tub, and Christina says she can not do it today. Nicole says it is now or never, and Christina says “Never.” Nicole warns her that he may kill her. Christina asks Nicole to help her, and she says she will do it. Nicole advises her to relax and be cool. Nicole goes upstairs and says it will be over in a half hour.
Nicole says Christina is resting, and she visits Mme. Herboux and M. Herboux (Noel Roquevert), who are listening to the radio.
Christina lets Michel in. He says he sneaked out of the school. Christina says she is not going back until after the divorce, and he learns she has a lawyer. He says he will go see the lawyer and puts on his coat. She says she lied about the lawyer, and he comes back. She says the divorce is better for both of them, and he asks if she knows its price. He says parents won’t trust her with their children, and she will go broke. He says he could have gone into business, but she wanted the school. She says he made her feel miserable. He sees the bottle, and he warns her about Nicole. He says he is replacing her and pours himself a drink. She causes him to spill it on his clothes, and he gets angry at her, slapping her. Then he drinks a glass, and she pours him another. He says they are leaving, but he staggers and asks for another glass. He asks where the princess is and lays on the bed. She takes off his shoes. Christina pours the rest of the bottle down the drain. Michel says he is tired, and they hear the bath running. Christina sees Nicole, who makes sure that Michel is unconscious.
Upstairs M. Herboux complains about the noise of the bath late at night, and he writes down the time. Nicole finds Michel’s return ticket and tells Christina to burn it. They carry Michel and put him in the bath. He opens his eyes, and Nicole pushes his head under the water. She tells Christina to bring in the heavy bronze, and Nicole puts it on his chest. She covers the tub with the tablecloth. Nicole says they have to be there for their alibi, and she is using the people upstairs. M. Herboux says she has not emptied the tub, and he will wait up until midnight.
The next day M. Herboux hears the bath draining, and he pounds on the floor. He helps Nicole carry the large trunk. It opens a little, and Nicole asks for string. They put the trunk in the car. As they drive, Christina says they are monsters. They stop for gas, and a drunk soldier asks Christina for a ride. She says no, but he climbs in the back. The gas station attendant makes him leave and sees water. Nicole toots the horn to get Plantiveau (Jean Brochard) to open the gate. They turn off the lights and park by the pool. They pull out the trunk, and Nicole stops Christina from coughing. They open the trunk, and a light from a toilet window shines on them for a short time. They dump the body in the water.
In the morning Nicole looks out the window and says they cannot see him from there. Teachers wonder where the principal is. Christina drills her class and looks out the window. She watches the pool man.
That night Christina tells Nicole he pretended not to see the body; but Nicole says it is at the bottom but will rise in a while. They are grading papers, and Nicole calls Christina a child for wanting to empty the pool. Then Nicole says she will have the pool emptied and hopes they won’t regret it.
The next day a ball is kicked into the pool, and the boys cannot get it. Nicole tosses something that lands in the pool, and a boy dives in to get it. He says it is too dark. They decide to empty the pool and tell Plantiveau to do it. Christina gives him money to do it right away. Boys in class realize that Christina is upset. She goes out to the empty pool and faints. Nicole takes care of her and says he is not there. Nicole gives orders to the other teachers. Dr. Loisy gives instructions to Nicole. She finds Christina praying. Nicole cannot figure out what happened to the body. Christina says she was crazy to listen to her. A man brings Michel’s suit from the cleaners.
They go to the address and ask who brought in the suit. They are given a hotel key and leave. Christina goes up to the room, and a janitor says no one has ever seen him there.
At home Christina accuses Nicole of the murder, and Nicole says how Christina helped. They argue, and Nicole stops Christina from calling the cops.
At the gate Nicole stops Christina from going to confession or to turn herself in, and she shows her a newspaper that indicates the body was found. Christina goes to the morgue and is asked to describe him. They bring out the body, and a man follows her. She says it is not her husband. The man shows her he is a policeman, Alfred Fichet (Charles Vanel), and they get into a cab. He asks why she did not go to the police. He is retired but says he will help find her husband. They go to the school, and he questions Plantiveau about the man. He notices they emptied the pool, and he talks with Nicole, who says he will never find him. He looks at things in his office and writes down his description. He checks his wardrobe to find out what he was wearing. Christina reacts to his identifying his suit. Nicole says she has a heart condition. Fichet says he will come back later.
In the evening outside a boy says the principal is punishing him, and Nicole slaps him for lying. Christina asks him to tell his story, and he says the principal took his slingshot. Christina says she is tired and retires with Nicole. They find the slingshot in the office. Nicole gives Christina a shot.
The doctor tells Christina not to go outside. The school poses for a picture. Nicole examines the picture that seems to show the principal inside the window. She tells Christina to get dressed. Christina says she cannot go out, and she tells Nicole to go. Nicole gets a suitcase and says goodbye. Christina moans in her sleep and wakes to see Fichet there. She says she is dying. He says he found him and that he is coming. She admits she killed him five days ago. She says what happened and says Nicole helped her. He advises her to spare her nerves and leaves. He says she will wake up acquitted. Outside Fichet asks students to show him the trunk. They go in the garage, and he finds the table cloth in it. Lights are turned out n the dormitory. Christina gets up and looks out the window, seeing lights go on. She goes into the hall and hears footsteps. She sees a light on and hears the typewriter. She enters the office and reads Michel’s name typed on the paper. In the bathroom she sees the body rise up out of the water and faints. Michel removes covers from his eyes and notices that she is dead. He lets Nicole in, and she kisses him. She says it is over this time. He says they are rich and can sell the school. Fichet comes in and says they will get at least fifteen years.
The next day the school is being closed, and people are leaving. The boy says Mme Delasalle gave him his slingshot, but a teacher says it is impossible because she is dead. The boy says he saw her.
This mystery drama intrigues the audience with its unusual ending. In retrospect the motivations make sense, though how it was all carried out strains credulity. In a deeper historical sense the drama represents an era when religion is no longer as important, and ethics may be declining. One woman is an ex-nun, and the other is apparently an atheist. Murder is treated as if it were a reasonable alternative to divorce.