The road to mediocrity is paved with good intentions, and indie icon John Sayles, director of “Matewan” and “Silver City,” can fall prey to an earnest idea without following through. His latest film, “Honeydripper,” a paean to the power of blues and rock ‘n’ roll in the postwar South, is a slog, replete with thudding characterization, bankrupt direction, and childlike plotting. And there’s not nearly enough electrified music for one to just wait out the rest of the movie.

“Honeydripper” lingers on a loose-knit swatch of social fabric in a rural town called Harmony, Ala. You’ll find juke joints, a segregated main street, cotton labor, a looming sheriff, but it’s all so luminously lit you know the movie will stay within its comfort zone. Danny Glover is Pine Top Purvis, owner of the Honeydripper Lounge, a barely patronized shack getting beat by the rockin’ establishment across the street. Late on rent and short on liquor, Pine Top fires the room’s majestic old torch singer and advertises play dates for regional legend Guitar Sam.

Whether Guitar Sam will actually turn up remains to be seen, but luckily a clean-cut guitar whiz kid (Gary Clark Jr.) blows into town — handsome feller, name of Sonny. Other than some halfhearted flirting with Pine Top’s wide-eyed daughter (Yaya DaCosta) who goes by “China Doll,” Sonny’s screen destiny is to wait diffidently in the wings till such time as he can swoop down, plug in, and electrify the day.

Stacy Keach’s ambiguously menacing sheriff stands in the way, detaining the non-resident for his cotton-picking gang, in one of the film’s barely written dramatic developments. Pine Top is also apparently facing strife with his wife (Lisa Gay Hamilton), who is wringing her hands over joining the church despite her unsaved husband. This conflict “erupts,” in one of the film’s many stagy encounters, and it’s about as effective as an awkward mutual revelation with her boss, an oblivious, tippling white lady played by Mary Steenburgen.

Mr. Sayles probably intends a sociological slice of the South, at a moment in the ’50s when music and society both underwent a heady ferment. But “Honeydripper” never sounds as if it’s made its way off the page, from the first harmonica whine set to a settling crane shot, to the futile, feeble “release” of the jamboree finale. Most scenes have a pregnant quality aiming for a slow summer’s day but more resemble polished but inert exchanges at a script reading. The editing is noticeably clunky, even counterproductive, from the outset, hampering the actors from building up any steam or vibe.

Any discussion of “Honeydripper” would be incomplete without a few more words about the music, but that’s the most frustrating feature of this leaden endeavor. Despite the presence of several musicians, get-up-and-shout energy doesn’t make nearly the impact it should, and even a blind streetwise bluesman (guitarist Keb’ Mo’) comes across as didactic. When Sonny hops atop a car parked outside the Honeydripper, the gesture is so muted that you feel vague confusion (“He’s … on a car”) rather than excitement. With a title like “Honeydripper,” you really have to give it up and turn it loose.

Mr. Sayles, who directed, edited, and wrote the screenplay, has long been taken as a model for steadfast indie filmmaking and sociohistorical conscience, but here he falls short of the standards his successes have set. This is neither the supercharged moment of rock’s birth, nor, thanks to the director’s uncharacteristically soft-pedaled politics, much of a study in race relationships. But that shouldn’t prevent you from giving any number of other titles under the Sayles label a spin on your DVD jukebox.