The film begins with Alphonse Tram (Gérard Depardieu), a less than gregarious character, idly chatting to an accountant who is travelling home very late (they appear to be the only ones on the La Défense metro station platform, but look closely...). The accountant, a man of orthodox social outlook and standing is disturbed by and fearful of this rambling loner, more so when Tram attempts to give him his bloodstained knife (in order to reduce the chances of him "doing something silly..."). They argue and the accountant puts the knife on a seat a few feet away behind them. They argue some more and then notice the knife has disappeared. Later that night Tram discovers the same man in a subway leading from another metro station lying down with the knife stabbed into his stomach: he has no explanation to the police inspector Bernard (Blier) he reports it to as to how it happened. He speculates, perhaps unwisely but without caring for the potential consequences (as in Camus' L'Étranger) , to the police inspector that it was his own knife that killed the accountant: the police inspector, irate at having to consider a complex case while off-duty pushes Tram out of his apartment saying he has a bellyfull of murders all day and doesn't want another to deal with. This sparks off a series of bizarre occurrences around the city as Alphonse's wife is killed, and the perpetrator (Jean Carmet) who confesses to the murder is seemingly taken light heartedly by the police officer and Alphonse himself. 

Source : Wikipédia