After the birth of the blues and the blues' subsequent marriage to rhythm, could the birth of rock 'n' roll be far behind?

Director John Sayles does the midwifing in his semi-historical fable "Honeydripper," so named for the once-hot, now moribund lounge in which its primary musical action takes place.

It's 1950 in the halcyon town of Harmony, Ala. -- so halcyon, it's asleep. The Honeydripper, run by Tyrone Purvis (Danny Glover), has more bills than customers. Big gamble: Tyrone lays off longtime blues singer Bertha Mae (Dr. Mable John) and announces the appearance of famous "Guitar Sam" in a one-night stand to save the club.

Meanwhile, drifter Sonny Blake (Gary Clark Jr.) shows up with what looks like a guitar case but no ordinary-looking guitar inside. It has wires and a plug, and Sonny needs an outlet in more ways than one.

A black man can easily run afoul of the law in this redneck of the woods, not so far from New Orleans as the Jim Crow flies. Sheriff Stacy Keach provides Sonny with a jail cell (and cotton-pickin' sentence) for vagrancy.

Guitar Sam fails to show. Tyrone springs Sonny to be a last-ditch sub but is very nervous about the nature of that guitar: There's no hole in the middle. Electrifying one instrument, let alone a whole blues band, will totally upset the balance, not to mention the acoustics. The music itself -- and a new musical style -- will have to change and conform to the technology.

"Honeydripper" is Sayles' 16th film, most of them explorations of American subcultures at pivotal moments: "Return of the Secaucus Seven" held a bittersweet reunion of '60s activists; "Matewan" chronicled a 1920s West Virginia coal strike; "Eight Men Out" was a nice autopsy of the 1919 Black Sox scandal.

Since there wasn't really a single event-based moment when R&B, Delta blues, gospel, jazz and country merged to form R&R, Sayles invents it -- or the cusp of it -- for us: Before the electric guitar, piano ruled. Afterward, the war for instrumental dominance between piano and guitar (Fats Domino and Jerry Lee Lewis vs. Chuck Berry) was won by the challenger.

There are racial and relational issues involved as well, and, all in all, more discord than harmony in Harmony. A fine performance is delivered by Clark, the charismatic newcomer, as Sonny. Versatile Glover, with his mesmerizing soft rasp of a voice, is as nuanced as always here. Supporting actors and singers (Dr. Mable John, Yaya DaCosta, Keb' Mo', Mary Steenburgen and Keach) are uniformly capable. Some of the terrific songs in the piece were written by multi-threat writer-director-editor Sayles himself.

From the gospel choirs and revivals to the cotton fields and the dark bar stages, everything in this picture is atmospheric, engaging and earnest to a fault. Only trouble is, at two-plus hours, it overstays its welcome by about 20 minutes.

As all good rockers know, it's better to leave 'em wanting a little more than less.

Opens Friday at Harris Theater.

• Coming tomorrow -- an interview with director John Sayles.