He stumbles upon a top secret experiment by a mad scientist (Bill Raymond) working on regeneration (you can see why the military would be interested in this) and begins growing younger. The government is interested in turning him into a guinea pig so they can learn the process by which the reversal of aging occurs. The movie is essentially about his run from the government while being pursued by vicious killers from various federal agencies.
The plot I thought was pretty well executed (except for the ending) and the effects are decent. I found Williams' wife (played by Frances Sternhagen) rather annoying, both in terms of character and her portrayal by Sternhagen. The side effects of messing around with regeneration is not addressed properly. There is some decent acting by Ed Lauter and Felicity Huffman who play government agents who turn against their government to save Williams life.
I am not necessarily a big fan of Stephen King movies, since they lack the thoughtfulness and imagination of his books, but I'd say this movie invites some provocative thoughts. This is a movie worth renting, even though it could have used a bit of editing.