A heavily didactic tone and the weight of good intentions bleed much of the dramatic life out of Gary Ross’ well-meaning but increasingly turgid docu-drama about Newt Knight, a deserter from the Confederate army who, between 1862 and 1865, led a revolt by poor white farmers and runaway slaves against the entrenched powers that were oppressing them both in southeastern Mississippi. From a base in the area swamplands, Knight forged a movement that defeated local military contingents, retook goods they had requisitioned and distributed them among his adherents, and gained effective control over several counties (the most notable of them Jones) adjacent to the Alabama border. Unfortunately, the post-war era of Reconstruction reversed much of what his cross-racial populist policies had accomplished.

As might be expected, Knight remains a highly controversial figure. There are those who hail him as a hero, a second—and in some ways more successful—version of John Brown whose progressive views were way ahead of their time (as shown in his marriage to an ex-slave and his championing of African-American voting rights) and who proved a master strategist as well. Others dismiss him as a traitor to the Southern cause, a simple criminal on a large scale. Ross clearly falls into the first camp: as played by Matthew McConaughey, Knight may be dressed in nineteenth-century garb rather than shining armor, but he still fulfills the promise of his surname. And those arrayed against him are villains—the wealthy elite, brutal soldiers, and racist civilians.

Knight’s admirable character is demonstrated in the film’s opening sequences, during his service on the battlefield in 1862. Though he grumbles over the inequity of the Confederate draft—the sons of the rich cotton-growers are exempt, depending on the number of slaves their fathers own—he’s a dedicated medic on the field, carting wounded men back to the makeshift hospitals where, of course, the officers get preferential treatment. What finally causes him to desert is the arrival of his young nephew Daniel (Jacob Lofland, who previously appeared with McConaughey in the superior “Mud”), a terrified boy who’s been forced into the ranks and is abruptly killed. Newt determines to take his body back home for burial, and finds that his neighbors are being robbed of their crops and livestock by Confederate requisition forces under the direction of loathsome Lieutenant Barbour (Bill Tangradi). After he stands up for them, he’s pursued as a deserter and forced to flee into the swamp for safety, where, thanks to the intervention of the plantation house slave Rachel (Gugu Mbatha-Raw) who’s earlier helped to nurse his young son by Serena (Keri Russell) back to health, he finds refuge with a group of runaway slaves headed by Moses (Mahershala Ali).

It’s not long before Newt and Moses decide to lure local lawmen into the swamp to ambush them. Soon other deserters, including Knight’s old battlefield pal Jasper (Christopher Berry) join them, and they form a quasi-military company that stop Barbour’s caravans and liberate the goods they’ve requisitioned. The lieutenant’s commander Colonel Hood (Thomas Francis Murphy) responds by burning farms and hanging a few of Knight’s followers who accept his phony offer of amnesty. That leads the rebels to mount an assault against his troops and seize control of the locality. Reinforcements force their retreat after German Sherman refuses any serious Union support, but Knight declares the area the titular Free State. Soon, however, the war ends, and the final third of the film covers in thumbnail-sketch fashion, combining dramatic recreations with documentary-style addenda, the unfortunate events of the post-war period, when initial progress in emancipation was derailed by the rise of the Klan and the development of the Jim Crow system.

To demonstrate the long-term effect of these changes, the narrative is periodically interrupted by scenes of a 1948 trial in which Davis (Brian Lee Franklin), a descendant of Newt—by either Rachel or Serena, since they both lived together with him after the war—is charged with miscegenation for marrying a white woman.

The story of Newt Knight is a fascinating one, and the first ninety minutes of Ross’ treatment—especially the initial battleground sequences—are quite compelling, even if they fall into a standard hero-vs.-villains, Robin Hood mode. McConaughey, with a scraggly black beard, brings the simmering intensity of an avenging angel—as well as occasional flashes of his trademark charm–to Newt, though the script gives the character relatively little nuance; his early scenes with Lofland are especially strong, though he holds one’s attention throughout. The remainder of the cast ease professionally into the fairly stereotypical roles the script has crafted for them, but only Ali makes a powerfully positive impression (Murphy and Tangradi are memorable too, but because of their Snidely Whiplash tendencies).

The final fifty minutes, however, grow increasingly dutiful, the cinematic equivalent of a Power Point classroom presentation on Reconstruction in which only a few moments—like the outcome of Moses’ attempt to register black voters—really hit home emotionally. Nor do the interruptions related to the 1948 trial work, because they’re clumsily staged and poorly acted. The Louisiana locations are nicely employed by cinematographer Benoit Delhomme, and production designer Philip Messina, set decorator Larry Dias and costume designer Louise Frogley have done a fine job in giving the images an authentic feel. But it’s not enough to compensate for the script’s deficiencies.

Newton Knight is certainly a figure of considerable historical interest, and it’s understandable that filmmakers should find him an intriguing subject. (Indeed, George Marshall’s 1948 “Tap Roots,” with Van Heflin, was adapted from a novel by James Street inspired by his life.) But he was also a very complex individual, and “Free State of Jones” simplifies and condenses so much about Knight and the movement he led that it can best serve as an invitation to read about him. A bit of research will reveal how conventionalized Ross’ treatment actually is.