It's a dark and compelling story of two working class women in their 30s, both of whom have things in their past that eat quietly away at them, though there's little on the surface that calls attention to their troubled inner lives. But when they meet and, over time, become friends and gradually share with one another their innermost secrets, their mutual influences on one another are such that their smoldering resentments burst out into violent deeds with a vengeance. But here the vengeance wreaked is against those perceived as class rather than gender oppressors, though, of course, the afflictions of these women at the bottom of the economic heap are gender specific, those of a daughter forced to care for her sick and demanding father and of a single mother working two jobs to support her daughter. But we don't know these things about Sophie and Jeanne at the beginning of the film. Melinda is the social conscience of the family and constantly throws it up to them when they treat Sophie in demeaning or thoughtless ways, but, on the whole, they think of themselves as good people treating their help kindly.
Nothing seems out of the ordinary here.
Except for the skillful ways Chabrol has of creating a sense of ominousness over the whole business. A good example is the scene where Catherine goes to meet Sophie's train on her first day of work. Needless to say Sophie does not get off the train. But the camera finds her in a long shot, standing on the next track silently watching Catherine watch for her, and a little tiny ominous chill runs down your spine. Later it seems we've found the clue to Sophie's oddness.