Claude Chabrol - Que La Bête Meure (1969)

A car runs over the boy Michel Thenier while he is crossing the street in his calm village in Bretane, and the hit and run driver flees without any witness. His grieving father Charles Thenier becomes obsessed to find the reckless driver that murdered his only son to kill him and spends his time writing his diary and running his own investigation in garages and junkyards. One day, he learns by chance that the actress Helene Lanson might be the responsible for the death of Michel. Charles travels to Paris posing as screenplay writer, introduces himself using his pseudonym of Marc Andrieux and seduces Helene with the intention of killing her. However, Helene says that her family is from Brittany and her brother-in-law Paul Decourt owns a garage in Quimper. When Helene invites Charles to travel to her sister's house to know her family, Charles finds that Paul is a despicable man with a disease in the intestine, hated by his family and his own son Phillippe Decourt wants to kill him.