Tanguy is 28 years old and still living with his parents. They think it's time he moves out. He doesn't, so they hatch a plan...

Witold just failed his law-school examinations and Fuchs has just quit his job at a Parisian fashion company. Arriving for a few days away at a so-called family guest-house, they are greeted by a series of unsettling omens: a sparrow hanging in the forest, then a piece of wood in the same condition, and finally signs on the ceiling and in the garden. In this guest-house there is also a baleful mouth, that of the maid, and a perfect mouth, that of the young woman of the house with whom Witold falls madly in love. Unfortunately, she has just married an architect of the most respectable sort. But is the young woman equally respectable? The third hanging, that of the cat, is Witold's doing. Why did he do it? And above all - will the fourth hanging be that of a human?

Doctor Knock is a former thug who has become a doctor and arrives in the small village of Saint-Maurice to make his fortune according to a particular method. It will make the villagers believe that they are not as healthy as they might think. It is thus that he will find in each one an imaginary symptom, or not, and thus will be able to exercise his profession lucratively. Under his seductive looks and after gaining the confidence of the village, Knock is on the verge of achieving his ends. But his past catches up with him and an old acquaintance disrupts the doctor's plans.