In a small French town, filmed in Valençay, the deputy mayor Pierre is having an affair of snatched meetings with Lucienne, the mayor's wife. So that they can be together more often, Pierre poisons his sickly wife without anybody suspecting and Lucienne then starts visiting his house at night. Her husband Paul, aware of what's happening, blackmails Pierre into supporting a dubious land deal. To escape this, the two lovers decide to get rid of Paul, who they kill on a lonely road and set fire to the car. Under orders from Paris, the police treat it as an accident, until they receive a letter from Lucienne's teenage daughter who has deduced the whole story. When arrested, the guilty couple do not deny their crimes. Incredulous, the police inspector asks them; "Why didn't you just leave? Go and live somewhere else?" Bemused, they reply; "We never thought of that."