Short plot synopsis from Wikipedia. A glamorous American widow marries a young Englishman in the South of France on the spur of the moment, they go to England to meet his parents: the mother-in-law takes a strong dislike to their new daughter-in-law, and a battle of wits ensues. However, the American girl has a dark secret in her past, while the father-in-law finds her a breath of fresh air.
Based on the 1924 Noel Coward play, the action moves along briskly enough: we sense at the end that all the right guys/gals get their gals/guys - or at least have a fighting chance; while the plucky, modern, American beauty gets one over those stuffy twenties English aristos.
Something for all of us then.
American beauty and father-in-law
Colin Firth, playing the father of the all-too-juvenile English aristo-husband, is at that awkward age: too old now for the lead romatic hero, while not aged enough to properly inhabit the previous generation. I guess this alone explains why he gets the best outcome of all in the final reel.