As the subtitle of this seven-film series' underlines (The Blues: A Musical Journey), the story of the blues is the story of transience, of exile. Even its documentation is a matter of hitting the road, as pioneered by John and Alan Lomax for their seminal Library of Congress recordings in the 1930s. Scorsese and budding bluesman Corey Harris follow the story back to its source, a voyage that ends not at the foot of Highway 61, but back across the Atlantic, in the former slave-farming grounds of Mali and the Niger delta. Salif Keita and Ali Farka Toure testify to a common inheritance, and the film illuminates how trace inflections of African fife-and-drum rhythms survived even a pre-Civil War prohibition on black drumming in the American South. It's a little sketchy, but right on the money.