We he comes out of the coma, he can't move anything but his left eye lid (this apparently is called locked in syndrome), which he learns to communicate (and eventually write a book) with. This is where the film starts. We see the world through his left eye for the first 20 minutes, which was a little creepy (people invade his space I lot) but clever at the same time.
As the film develops we get to see him from the outside and learn what his life was like before he had the stoke, but whenever he tries to communicate, we usually enter his head.
Apparently this is based on a book of the same name. It's a good film, but I don't think I could read the book, it'd be far too much like heavy going I think.