Les Vécés étaient fermés de l'intérieur (1976)
Directed by Patrice Leconte

Film Synopsis

Gaspard Gazul is a Parisian bus conductor who meets a nasty end one morning when he nips into a public convenience to relieve himself.  The instant he flushes the toilet there is an explosion and the poor man is instantly obliterated.  Who would do such a terrible thing?   This is what police Superintendent Pichard and his colleague, Inspector Charbonnier, have to discover as they begin their investigation into this most peculiar of murders.

One odd feature about the case is that the toilet in which Gazul met his end was locked from the inside, so it seems unlikely that anyone could have inflicted the mortal blow directly.  There seems to be nothing in the dead man's past that could account for why he should be killed in such a bizarre fashion.  But by examining Gazul's post Pichard comes across a letter that might offer a valuable clue as to the identity of the killer...