One of the best Miyazaki moviesm, in my personal opinion. But when you look at their history, studio Ghibli produced one genius movie after another, so if you didn't watch any, you gotta get them and watch them!
So Princess Mononoke is a bit different, of course like most of their movies they have a theme, and this movies theme was environment and pollution, the destruction that human nature lives behind.
The story covers a young prince from a village who goes on a big journey. I will not tell you what exactly he meets on this journey, but I will tell you that he is the kind of character that is right there in the middle between loving animals and loving humans, hence he is the one who helps both sides.
I really enjoyed this movie and it is really a piece of art. If you haven't watched it yet, go do it, here is the trailer: