La Règle du Jeu (‘The Rules of the Game’, Jean Renoir, 1939)

Sincere people are such bores

La Règle du Jeu starts with a simple affair: young adventurer André Jurieux has a crush on Christine de la Cheyniest, whose husband, Robert, thinks it's a charming diversion but wants it to end. From there on out the film snowballs more and more different characters into the proceedings, employing an 'upstairs, downstairs' structure you may recognize from Downton Abbey. Most of it takes place in a mansion on the countryside, where masquerade revelries and ballroom games take up much of the time.