This lively, no-frills documentary explores the evolution of black gospel quartet music and its contributions to the emergence of rock ‘n’ roll, starting with the ensemble-oriented harmony singing of the jubilee style in the 1930s and leading into the harder sound of the ’40s, when charismatic lead singers turned in passionate, virtuoso performances with call-and-response patterns and plush backing vocals. Veteran performers and historians explain the hardships visited upon the musicians by racism, their struggles to stay on the road when gigs offered only bare subsistence, and the way evolving public tastes lured some of the greatest voices, such as Sam Cooke and Lou Rawls, to switch to secular music. Director Robert Clem generously incorporates uninterrupted vintage performance footage by the likes of the Fairfield Four, the Highway QCs, and the Blind Boys of Alabama.