Back down home, somewhere on the Mississippi River on the Arkansas state line, is where the very young writer/director Jeff Nichols (“Shotgun Stories,” “Take Shelter”) grew up — and it’s obvious. I grew up nearby in Missouri, and I can tell you that in “Mud” that’s the river, and those are the poor southern Scotch-Irish, full of dreams and absent a plan. These are folks who don’t live on the edge, but far below it on the rocks of calamity canyon.

“Mud” is a coming-of-age story about two boys, Tom and Huck — well, a modern variation. It’s Ellis (Tye Sheridan) and Neckbone (Jacob Lofland) and their river, their dirt bike, flat bottom boat, their strip malls and hillbilly high school. They are surrounded by the danger of low expectations, bottom-line education and a future that pales with each setting sun.

Ellis’ mom and pop are poor folks with good hearts (a very good Ray McKinnon and Sarah Paulson).
Neckbone is a discarded soul, being raised by his uncle (Michael Shannon) who ekes out a living by scouring the river bottom for what appear to be oysters.

True to Twain and the lore of the river, the boys go off on their adventure to motor out to an island in the middle of the river where they know a boat, thrown by floods and storms, is high up in a tree, like a tree house. They have big plans for this boat, but there’s another fella who has his own plans and that would be “Mud,” played to bottom-mud perfection by Matthew McConaughey. 

Mud, they discover, is on the run, been on it since he went and killed a bad guy in Texas who was nasty to Mud’s childhood sweetie, Juniper (Reese Witherspoon.) We soon learn that Juniper has passed around her favors in every juke joint from Arkansas to Amarillo and back to home.

But now both lovers are back in the hometown, with Juniper in a motel waiting for a call, and Mud up in his island treehouse boat, trying to stay out of sight of the lawmen who are blocking the highways out of town.

To add to his worries, the papa of his victim (old-time backwoods icon Joe Don Baker) has arrived in town with his own posse of low-lifers on his payroll. Papa has no need for the law — he’s rich and vengeful and has come for blood.

Thus, with everyone out looking for Mud, and unaware that he’s out on that island with a bonfire each night, we’re looking at a down-south “Great Expectations.” Mud is our accidental killer with a big heart and he suckers the boys, who have fallen in love with this wild and crazy no-good hero, into bringing him more food and parts to get the boat going, so he can cruise down to the Gulf and off to Mexico. 
Tall Tom Blankenship, a mysterious loner (a great and welcome Sam Shepard) folds in here  as an old friend of Mud, who lives across the river from Ellis. Mud tells the wide-eyed boys that Tom went to Yale, and was a sniper assassin for the CIA. Mud is given to tall tales, but that sniper story will come into fruition in the finale.

Director Nichols fleshes out the plot with the boys aiding and abetting, stealing parts and equipment from local junkyards, running notes back and forth from the motel to the island and generally trying to add their own paragraphs to the growing legend and great escape of our flawed hero.

Nichols’ script is tinted with bits of Tennessee Williams, Mark Twain and old John Ford river sunsets. His Juniper, Witherspoon, isn’t really bad enough for the sweetheart-gone-wrong part. It’s my secret idea that Jennifer Lawrence would have gotten it right. Reese just looks like she was in town for a photo op and got the job. She barely works up an acting sweat.

The boys are clearly at the heart of the story, and Nichols lucked out with them both. Sheridan (“Tree of Life”), as Ellis, is a born actor with a career waiting to happen.

His heavy emotional scenes with his parents, and with McConaughey, show a sharp inchoate talent. 
In his very first part, Lofland, as “Neckbone”  seems to have just stepped out of a strip mall drugstore in Arkansas, as well he might have.

McConaughey, a son of Uvalde,Texas, clearly has the dirt-road credentials to bring this backwoods fugitive to life — and well he does. With mud-soaked hair, sunburned skin and one shirt to his name, he charms the boys with tales of snakebites, jailhouse tattoos and misty-eyed memories of his dusty backroad love affair with Juniper. It seems then that they will do anything for him — and they do — until true life breaks over them like a river storm.

“Mud” is a good solid movie, not great, not fierce. It moves slowly like the river that runs through it. But like that river, a lot of emotional debris, chunks of soul and pieces of broken hearts pop to the surface.

J.P. Devine is a former stage and screen actor.