From Yahoo Movies:
Marc and Agnes are an attractive, successful Parisian couple, who have been together for many years. One evening, before joining friends for dinner, Marc decides on a whim to shave off the mustache he's worn all of his adult life. He waits patiently for his wife's reaction, but neither she nor his friends seem to notice. Stranger still, when he finally tells them, they all insist he never had a mustache. Is Marc going mad? Is he the victim of some elaborate conspiracy? Or has something in the world's order gone terribly awry?
I may want to get this on DVD, since I won't be able to go to the movies and see it.