The story of sisters Christine and Lea Papin, two house-maids who, in Le Mans in February 1933, brutally murdered Madame Lancelin and her daughter Genevieve. Sent out to work at an early age, Christine and Lea are employed as maids by the Lancelins and treated well. The girls share an attic room and the relationship between the older, more domineering Christine and her much younger, rather simple-minded sister Lea turns into a sexual one. One day, when the mistress of the house and her daughter Genevieve return unexpectedly from a day trip, they are surprised to discover that the two girls are already in bed. Madame Lancelin demands an explanation from the semi-clad older sister. Christine becomes hysterical, grabs a knife and begins to stab wildly at both mother and daughter. When Lea arrives on the scene, Madame Lancelin is already dead. Shocked by what she sees, Lea now moves in on the act, killing Genevieve.