Still of Matthew McConaughey, Tye Sheridan and Jacob Lofland in Mud. (Jim Bridges)

Matthew McConaughey has been on an extraordinary run of late, turning an impressively versatile hat trick in “Bernie,” “Magic Mike” and “Killer Joe” while proving that, rather than the tabloid punch line or rom-com sellout he seemed destined to be at one time, the boy with the bedroom eyes and bong-hit grin is a real actor, after all.

McConaughey’s low-key comeback continues with “Mud,” in which he plays the title character as a modern-day cross between Boo Radley and Robert Mitchum’s Max Cady. As the slippery central figure of Jeff Nichols’s richly observed coming-of-age fable, McConaughey injects a note of danger into a bayou noir story of youthful ad­ven­ture that manages to be lyrical and sobering at the same time. Aided by extraordinarily assured performances from young co-stars Tye Sheridan and Jacob Lofland, McConaughey defines his own version of a familiar Southern character — the frightening Other who can strangle as easily as save — as an enduring archetype rather than irritating stereotype.

As eye-catching as McConaughey’s performance is — thanks, in large part, to a leathery suntan, body-wrapping tattoo and snaggly prosthetic teeth — “Mud” really belongs to the 14-year-old boys who cross Mud’s path with memorably fateful results. Ellis (Sheridan) and his best friend, Neckbone (Lofland), live in a poor Arkansas community just off the Mississippi River, where Ellis lives on a houseboat and helps his father sell catfish door to door. When the boys sneak off to a nearby island and happen upon a boat lodged high in a tree, their imaginations are sparked: Soon they’re inspecting it for future use as hideout, clubhouse and all-around perfect means of adolescent escape.

Their plans are foiled when they meet the lone inhabitant of the island — Mud, whose reasons for being there are as mysterious as how he arrived. Nichols brings the same observant ear and sharp eye for atmospherics to “Mud” that he brought to his thriller “Take Shelter” two years ago: The film is drenched in the humidity and salty air of a Delta summer, often recalling the musical, aphoristic cadences of Sam Shepard, who happens to appear in a supporting role.

Easing in for close-ups on Sheridan’s and Lofland’s open, expressive faces, Nichols hits a prime balance between naturalism and more fantastical elements in telling their story, in which menace and tenderness coexist with finely tuned equipoise. Ellis’s cramped, cluttered houseboat, in which his parents continually argue over whether to move into town, and Neck’s far more precarious life with his young uncle Galen (Michael Shannon), are depicted with convincing realism. But then Nichols follows the boys to the wide river, and that gloriously impossible boat suspended in the tree, and “Mud” lifts into something more mythic and giddily fanciful.

Sarah Paulson and Ray McKinnon bring a sense of grounded plausibility to their roles as Ellis’s unhappy parents, and oddly enough, Reese Witherspoon’s recent brush with the law in Atlanta actually helps make her more believable as the hard-bitten femme fatale who propels Ellis and Neckbone into learning unwelcome lessons about love, loyalty and disillusionment. (Those lessons culminate with the film’s only misstep, a preposterously staged climax that temporarily shatters the convincing universe Nichols has created up to that point.) With only one or two important exceptions, everyone in “Mud” is fundamentally good, bringing weary, weather-beaten integrity to the day-to-day business of doing the best they can.

Simple values — hard work, honesty, trust, basic decency — inform everything about “Mud,” from its story to its aesthetic. That simplicity makes Nichols’s film something of the anti-“Beasts of the Southern Wild,” last year’s art-house darling that depicted a similar world and its marginal figures with stylized — and troubling — exoticism. Rural poverty isn’t treated romantically or, conversely, with a superior air of judgment in “Mud”; Ellis’s mother is no less sympathetic for wanting to move to the city than is his father for tenaciously defending their disappearing way of life. This is where a filmmaker’s taste and reflexive sense of balance makes all the difference. Southern culture may be on the skids in “Mud,” but Nichols’s sensitive portrayal is gratifyingly on the level.

PG-13. At area theaters. Contains some violence, sexual references, profanity, thematic elements and smoking. 130 minutes.