If “Beasts of the Southern Wild” and “The Paperboy” whetted America’s appetite for Gothic tales of the working-class South, “Mud” may be just the offering to satisfy those cravings.

Writer/director Jeff Nichols creates richly realized characters in a tale that moves like a cottonmouth viper, advancing slowly until it strikes with sudden violence.

Fourteen-year-old Ellis (Tye Sheridan, “The Tree of Life”), who lives on a funky Arkansas houseboat, inhabits a present-day world where few relationships have a solid foundation. He has good reason to worry that his parents (Ray McKinnon and Sarah Paulson) are headed for divorce. His best buddy, funny, foul-mouthed Neckbone (Jacob Lofland), is being raised in a trailer park by his reprobate uncle Galen (Michael Shannon), whose bedroom should have a turnstile.

Turning their backs on their town’s strip malls and housing developments, the boys motorboat to an isolated island in their slow-moving section of the Mississippi. Discovering a small cabin cruiser docked overhead in a tree, they aim to make it their clubhouse until they stumble onto Mud (Matthew McConaughey), a rough, gun-toting drifter using it as a hideout.

Mud treats the boys like trusted younger brothers, admitting that he’s on the run after killing a man who abused his childhood girlfriend, Juniper (Reese Witherspoon). Dispensing practical wisdom alongside iffy life advice he considers sage, Mud wins them over.

Ellis is particularly struck by Mud’s devotion to Juniper. He’s looking for a version of love that works, and the adults around him are bad examples. Ellis blossoms under Mud’s attention. He slugs a bullying older boy, winning the David-and-Goliath confrontation and gaining the attention of the high school’s teen queen.

The boys give their new friend the benefit of the doubt and help him prepare his escape to Mexico. They scavenge materials to repair and lower the boat. Mud also recruits Ellis as his messenger to Juniper, who is lying low in town, awaiting their getaway. Then the dead man’s powerful father (hulking bad old boy Joe Don Baker) rolls into town with a band of bounty hunters to take the fugitive, dead or alive.

The boys learn there are undisclosed sides to Mud’s story as he’s revealed to be both less and more than he appears.

Though the film has a spirit all its own, comparisons to Tom Sawyer and Huckleberry Finn are inevitable. Nichols even has a mysterious key character (played by Sam Shepard) named Tom Blankenship, after Twain’s real-life model for Huck Finn.

Like its inspirations, “Mud” grapples with mythical ideals of American manhood and captures the region’s pastoral beauty and ugly civilization. It’s also chock full of indelible characters; even the walk-ons are three-dimensional.

McConaughey is deeply committed in his work as the rustic Romeo, a gnarly, morally confusing ne’er-do-well whose unpredictability keeps us guessing. With greasy hair, a chipped front tooth and an air of regret, he’s traded away his heartthrob image for a terrific role in a fine, resonant movie. It’s a heck of a good deal.