I can't really describe the plot, simply because there are so many little tiny stories all going on. Basically, the movie is a murder mystery at a country house- but the murder doesn't take place until almost 3/4 through the movie.
No, the big thing about "Gosford Park" is the intertwining of the stories that each of the innumerable characters has, and the struggle between class differences. And, in only 2:18, Altman is able to successfully tell each character's individual story fully and completely. Each character is developed brilliantly, each scene is perfectly needed, and every performance is brilliant.
I particularly loved Maggie Smith and Helen Mirren of course - the two Oscar nominees that came out of this movie. Smith is delightful and irresistible (think...Judi Dench in "Shakespeare in Love") and Mirren is so brilliantly subtle. But there are so many other noteworthy performances. Tom Hollander and Emily Watson particularly stood out to me, and the magnificent Kristen Scott Thomas proves her prowess once again.
Anyway, I unfortunately can't say much more about the movie - it's really a film that I feel I need to see again and, on top of that, that cannot really be described fully in a meager blog post. I will say that you must be prepared - it's slow moving, though oh so rewarding. I don't know if I absolutely adored this film with my heart, but from a film perspective, it's easily one of the most well made and intriguing movies I've seen in quite a while. I'm still amazed at how easily it put together all those characters.