"Mud" serves as the third film for writer-director Jeff Nichols, but this one comes with something new: expectations.

His first film, "Shotgun Stories," got his name out there and established his relationship with currently in-demand character actor Michael Shannon before his "Revolutionary Road"-aided breakout. Then came 2011’s "Take Shelter" – also starring Shannon – a borderline masterpiece and easily one of the best films of that year.

And when the words "borderline masterpiece" are used to describe one of your movies, it’s safe to assume hopes will be astronomically, unfairly high for your next project (Neill Blomkamp, director of "District 9," will know how this feels in about two months when "Elysium" comes out).

Now there’s the follow-up, "Mud," a grimy Mississippi River-based coming-of-age tale starring an unkempt Matthew McConaughey. It’s a modest, paced picture that serves as a worthy follow-up, moving like a raft floating calmly down the Mississippi on a lazy summer day and taking in all the character on the shore along the way.

McConaughey may get top-billing as the unfortunately named title character, but the film belongs to the young, remarkable Tye Sheridan, previously seen in "The Tree of Life." He plays Ellis, who alongside his friend Neckbone (Jacob Lofland), discover Mud, a mysterious loner hiding on a small, uninhabited river island in a dilapidated boat stuck up in a tree.

Mud assures the kids that he means no harm. He’s simply waiting for his long-lost loved one Juniper (a glamour-free Reese Witherspoon) to meet him on the island, and they’ll sail off together on the boat. He just needs some help delivering messages to Juniper and getting spare parts to get the boat up and running again.

The boys agree to help, but the more they find out about the mysterious stranger – from his estranged father (Sam Shepard), the local authorities searching for Mud and some intimidating gentlemen descending on the tiny town (led by Mitchell himself, Joe Don Baker) – the more dangerous protecting Mud and his ill-advised, obsessive quest for Juniper becomes.

The first half of the film focuses on the enigma who is Mud. Can Ellis and Neckbone trust this bizarre mix of contradictions? He’s a gnarly, nature-beaten man who also wears a sweaty, off-white collared shirt as some last vestige of civility. The police and suit-wearing gangsters tell the boys Mud has killed a man, but despite having a gun stuck in his pants, he never seems like a threat, and his motives seem innocent enough. It’s an intriguing, slow burn of a mystery that carefully unpeels the character underneath all of the rumors and grime.

Even as the truth unfolds, Ellis can’t help but be magnetized by Mud’s sweet but self-destructive desire for true love. After all, he’s experiencing his own first pangs of love for an older high school student, and it’s not like his near-divorce parents – or Neckbone’s womanizing goof of an uncle (Michael Shannon, tragically underutilized) – would seem to understand. No matter how strong the effort or the feelings, though, love is not as fair as either Ellis or Mud would like it to be.

Much like in "Take Shelter," Nichols displays a careful touch throughout "Mud." His eye isn’t showy but he makes the most of his muddy Southern landscape (a shot of gangsters gathered in a dark motel room is a rich wonder of framing and lighting) and perhaps most of all with the gritty local color.

Every rickety wooden house in the swamp and Southern drawl feels like it was pulled straight off the river. Nichols brings these small towns to life on screen, and they return the favor by giving his stories a kind of mesmerizing, low-key authenticity.

Nichols drops his characters into this unassuming world, and they fit right in thanks to the performances. McConaughey might seem a bit too handsome even under his dirt-matted hair, but the emotions all hit their marks. The supporting cast all blends into the story with ease as well.

The true find, however, is Sheridan. There isn’t a dishonest moment in his entire performance, as his sweet, innocent optimism slowly turns into anger as all of the love stories surrounding him – including his own – fall apart. Ellis is maturing, and his world is complicating fast, but he’s still a kid, wild, free, adventurous and a whirlwind of confused emotion. Sheridan aces it all with a natural mix of composure and childlike earnestness.

Even when the guns come out for the film's finale, it’s an easy pleasure watching these characters grow and evolve with such heartbreaking and eventually heartwarming honesty. The same could be said for Nichols.