Making the preposterous look classy : Merci pour le chocolat

Claude Chabrol's "Merci pour le chocolat" (2000) would seem to have it all, a well-respected French director and writer (Chabrol), a classy star (Isabelle Huppert), and the quintessentially French quality of sophisticated, educated, mature adult characters (a la Renoir, Rohmer, and a million other), they are scientists and CEOs and concert pianists and they engage in easy repartee about their multiple marriages and exes and troubled teens all over drinks or espresso or simple but beautiful cafe food, great setting in a lakeside town in the Swiss Alps, even the business people have a cool and socially acceptable profession (they make Swiss chocolate), lots of info given right from the top in smart dialogue, in other words, completely civilized and smart, and yet, and yet - it's a lousy movie. Too bad, and I'm not sure what anyone could have done with this material, adapted from an (English?) potboiler whodunit, completely noncredible: in essence, the chocolate heiress/CEO (Huppert - Mika) controls the behavior of her stepson by lacing his evening chocolate with Rx, apparently she did away with her rival by the same way, leading to a fatal mountain car crash, now through a misstep she almost does in the wrong person, but, helas!, it lead only to a crash, not a fatality - she's a ridiculous femme fatale, and though the characters and setting are totally believable the actions of the heroine and thus the plot of the story itself is ridiculous - Chabrol can be blamed for choosing the material, even if he did an adequate job of making the preposterous look classy.