"Coup de Torchon" or "Clean Slate," which opened recently at the K-B Janus, is the official French entry in this year's Academy Award competition for best foreign-language film. That fact should not be construed as a recommendation. Maybe half a recommendation: The amusing aspects of the material undergo a lamentable change for the solemn at about the halfway point.

A stunted example of cultural cross-fertilization, "Coup de Torchon" derives from a 1964 American pulp novel, Jim Thompson's "Pop. 1280," published in a popular series of hard-boiled crime fiction. Director Bertrand Tavernier and screenwriter Jean Aurenche have transplanted Thompson's smalltown setting, Hicksville, to a fleabitten colonial outpost, Bourkassa, in French West Africa, circa 1938.

Philippe Noiret's character, Lucien Cordier, is a slovenly, cowardly cuckold obliged to muddle along as the town's solitary, ineffectual policeman. Nagged by his shrewish wife (Stephane Audran), who openly consorts with a pampered, good-for-nothing lover (Eddy Mitchell), described for convention's sake as her brother, and shoved around by the local criminal element (Jean-Pierre Marielle and Gerard Hernandez as a pair of pimps), Lucien is accustomed to avoiding trouble and shrugging off humiliation.

However, a brusque pep talk (reinforced by kicks in the pants) from a self-assertive colleague (Guy Marchand) in a neighboring town stirs a suppressed desire in the debased and abused civil servant to get some of his own back. The worm turns by becoming a furtive assassin. Because the victims appear to be eminently expendable wretches--the pimps and then a vicious local merchant whose long-suffering wife is Cordier's mistress (Isabelle Huppert's delightful performance as the beneficiary of this liberating murder proves the movie's only spontaneous source of fun)--the slate-cleaning has an irresistible humorous appeal, in the comedy-of-homicide tradition of "Kind Hearts and Coronets" and "The Green Man."

Although Tavernier's touch is never sprightly, there's no reason to apprehend "Coup de Torchon" as anything but a facetious parable of depravity for the first hour or so. Then, alas, Tavernier switches to his serious, guilt-mongering frequency, and Cordier's crusade is transformed into a parable of homicidal dementia, ironically anticipating the slaughter to come in a world on the brink of madness and war. In other words, he blows it.

Tavernier forces the material to navigate a turn that runs the plot aground, and your enjoyment with it. Always too coarse and poky for stylish comic satisfaction, "Coup de Torchon" ends up feeling like a peculiarly annoying double-cross, a movie that fails to play its premise conscientiously in either a comic or serious vein.