A vital musical ramble. Directed by Chris Hegedus and D.A. Pennebaker, this documentary shows us where some of soul's founders ended up after their moments of pop glory. The picture doesn't have a lot of structural unity, yet many of these ageless artists perform with a ferocity and funk that has only been amplified by the hard depths of their experience. As long as you're listening to Wilson Pickett, Sam Moore, or Memphis ''king'' Rufus Thomas doing ''Walking the Dog'' with a bawdy wink that belies his 82 years, it's one of the happiest movies around.