Respected professionally and socially, the widowed watchmaker Michel Descombes lives peacefully over his shop in Lyons with his teenage son Bernard, who works in a factory. One morning when his son has not come home, which is not unusual, police arrive and take Michel to see their inspector, who gradually breaks the news that the boy has killed a man and fled with a girl called Liliane.
Michel knows nothing of the victim, an unpopular security guard at the factory, or of the girl, who worked there. In shock, he naively tells intrusive journalists whatever they want to hear. Once calmer, he realises he must be on his guard and when two thugs break his windows, he and his assistant beat them up, throwing one in the river. He visits the old woman who looked after Bernard when his mother died and discovers that she is closer to the boy than he is. The inspector, hoping that Michel may lead them to the runaway pair, puts a lot of effort into winning his confidence.
When the police do trace the two to a town in the north, he invites Michel to travel up with him. Arrested, the young people are flown back to Lyon for trial and Michel hires a lawyer, who has his own ideas on defence. It is alleged that the murdered man demanded sex from Liliane or he would frame her for stealing. The inspector also thinks the crime was not premeditated but the court is not convinced, giving Bernard 20 years for murder and Liliane 5 for abetting. Michel visits Bernard in prison to tell him that Liliane is pregnant and that he, with her parents, will look after the child. The two discuss possible names.
