A weekend on the countryside for the English upper class turns sour when their host ends up dead. Of course, the death of the host actually happens more than halfway into the film, so it is VERY SLOW GOING until then. And then, assuming the audience has been paying attention, to them it is reveled the who and the why. Although it remains a mystery to all but 3 of the large number of characters.
The film really criticizes the upper class. The only likeable characters in the entire film are the servants.
I watched it because I had read somewhere that the music was quite good. And yes, a very good song is featured: "The Land of Might-Have-Been". But other than that (and perhaps the funny "And Her Mother Came Too"), the music, while beautiful, is entirely relegated to the background, and doesn't play a significant part in the movie. The mystery plot is the point of the movie, but it takes a long while to get there.
Gosford Park was a damn good mystery. It explored some really interesting themes with the 1930s British class system and is essentially a prototype of Downton Abbey, at least at the start. It was somewhat difficult to keep up with who was who at first because the cast is huge but you get plenty of time to learn about everybody and possible motives well before the actual mystery starts. There's just so many interweaving subplots and it all flows so well together, it's really impressive.