I will not superfluously add to the excellent technical description of the plot given by nenslo. But I beg to differ on the overall comical content of this comédie noire, which is much higher IMO, not to mention the very high caliber of most of the actors and actresses and the dynamic authority of the camera work. Even though some of the humor lies in physical situations and a bit of slapstick, most of it is in the text, those lines delivered full tilt (subtitles are essential here – even for a French spectator such as me – us contemporary audiences having grown somewhat alien to fast thinking and talking in English and French films!), as well as in some subtle political winks.
The character of commissaire Baudu (played by a young Michel Serrault), ever nostalgic of the Gestapo methods of interrogation from the collaboration days of WWII in France, is probably one of the main reasons why this film, in spite of the major presence of De Funès playing here his typical screen self with much gusto and brilliance, was accursed by the French society of the day, for which the collaboration was still a big taboo, or at least certainly not a pet topic for humor.
"Carambolages" probably owes a lot of its own vision of the modern corporate world and its cinematic treatment of the subject to Jacques Tati's films ("Mon Oncle", etc.). Conversely, the entire opening scene, a conference room projection of a publicity campaign film as seen from the Carambolages spectator's POV, cutting to the boss admonishing the execs over it and vehemently requesting instead something that will make the customers hopelessly, sickeningly addicted to the product, struck me as having been lifted almost literally by Richard Donner's "Scrooged", 25 years down the road, for its own opening sequence! As well, the explosive cigar box gimmick, prepared step-by-step before our very eyes and fiendishly efficient (in a rather terrible scene, including the ensuing panic of the fellow workers), not to mention the perpetrator's reaction to its imminent opening-activation, sure as heck recalls a Columbo entry entitled "Short Fuse", featuring Roddy McDowall!
