Proprietor Pine Top Purvis (Danny Glover) has everything riding on having a break-the-bank weekend, the only chance he’s got to save his club. He’s counting on a big-draw appearance by radio star “Guitar Sam,” though it’s the appearance of a young drifter (Gary Clark Jr.) sporting a homemade ‘lectric ax that’ll prove unexpectedly serendipitous. It’s a sturdy enough ticking-timeclock premise, serving as a foundation from which to survey the black life of Harmony, Alabama, AD 1950, as Pine Top’s story touches on tent revivals, cotton fields, and domestic-and-mistress interplay. Writer/director John Sayles takes a relaxed approach, letting characters congeal, and Glover is the keystone in an ensemble of very human performances. But that same leisurely attitude becomes a problem when the plot starts demanding attention again—the twists of the film’s final section will feel excruciatingly inevitable to anyone who’s seen a movie before, and the payoff isn’t there. We’re supposedly seeing the ground zero of plugged-in blues—of rock ‘n’ roll. But when it’s time to recreate that flashpoint moment, the performance (by Clark) lacks charisma—Sayles makes it pretty tough to buy that “Good Rockin’ Tonight” never sounded like this before.