Claude Chabrol - Les noces rouges (1973)

In a small town in France, the mayor Paul Delamare invites the leftist Pierre Maury to be his deputy mayor in his reelection. Paul is married to Lucienne Delamare and stepfather of Lucienne's teenage daughter Helene Chevalier, but sexually neglects Lucienne. Pierre is married to the weak, sick and bitter Clotilde Maury. Soon Pierre and Lucienne have a torrid lover affair during Paul's frequent travels to Paris. When Clotilde dies, the doctor believes it was a suicide, and the case is closed by the police. Paul proposes a dirty real estate business to Pierre to lure the inhabitants of the town and make lots of money, but Pierre is reluctant to accept the proposal. Pierre discloses to Lucienne that he killed Clotilde that was very ill, and they should take care with their encounters to avoid gossips and suspicion from the locals. When Pierre travels to Paris, Lucienne spends the night with Pierre, and when she returns home at 4:00 AM, Pierre surprises her since he has not traveled. He calls Pierre and uses his affair to force Pierre to accept the dirty deal. Now Paul will travel to Paris, and Lucienne asks him to go with him. While Lucienne is driving in the lonely road, she stops the car on the shoulder and Pierre kills Paul and burns the car to the ashes simulating an accident. The police clear Lucienne and close the case as car accident, but Pierre advises her not to see each other for a long period. Everything is perfect for the two lovers until something unexpected happens.