You will find no green trees, sunshine, or healthy, happy-looking people in Cormac McCarthy's novel The Road, much less anyone who even remotely resembles Charlize Theron, so it's a bit of a surprise to see them all in the opening frames of director John Hillcoat's movie adaptation. They turn out to be no cause for serious alarm. Hillcoat (The Proposition) and screenwriter Joe Penhall have, in fact, taken remarkably few liberties with McCarthy's post-apocalyptic best-seller, and where they have, the changes help bring to life the themes and the emotions that swelled beneath the terse prose and bleak narrative from the outset.

There's still plenty of bleakness to go around. The movie's action picks up with the novel's first pages as an unnamed man (Viggo Mortensen) awakes, wrapped in rags and plastic tarps, in a rude, rain-soaked camp next to an unnamed 11-year-old boy (Kodi Smit-McPhee). The unspecified catastrophe artfully hinted at in the prologue has blocked out the sun, snuffed almost all animal life, and killed off all but a handful of human survivors, as a brief voice-over reveals. The man is shepherding the boy, his son, through society's wintry blasted ruins on foot toward the coast, where some unspecified hope awaits. They are forever searching for evermore scarce food and water and always on the lookout for the armed gangs who have survived by killing and eating the weak. The man carries an old revolver in his belt, loaded with his last two bullets, held in reserve so they can put themselves out of their own misery if they get caught.

How the movie unfolds from there follows the book's footsteps almost exactly. Traveling through the unrelenting gloom, they encounter cannibal gangs and witness the absolute worst people can do to each other. They face starvation and stumble across a mother lode of supplies. They encounter a few other isolated travelers (including The Wire's Michael K. Williams and a nearly unrecognizable Robert Duvall). But despite the ready-made narrative engine, the road itself takes something of a backseat in Hillcoat's movie.

This Road is, in many ways, about the difficult transition between one type of world and another, and the addition of Theron playing the man's wife, a barely mentioned background presence in the novel, strengthens the theme. As revealed in flashbacks, she is pregnant with the boy when the cataclysm strikes; during labor, she howls as if it's not the contractions that torture her, but the pain of knowing what her child faces. As the flashbacks continue, she makes it plain that she doesn't want to face this new world either. Back in the present, the man keeps himself alive to keep their son alive, but he is crippled by his memories and reminders of everything he has lost. Meanwhile, by subtly linking the boy with the fading artifacts of a world that died before he was born - a stuffed deer head, an ATM sign - Hillcoat makes clear that, as horrible as it is, this new world is the boy's by birthright.

And he is in many respects better adapted to deal with it than his father. Mortensen's character is no post-apocalyptic ass-kicker. His performance is defined by delicacy and quietness, a kind of ingrained hesitancy, which makes the straits they find themselves in feel even more desperate. He instills in the boy the notion that hope exists as long as they're alive, and that they're the "good guys," as opposed to those who'll stop at nothing to survive. It's the boy, however, who helps keep their humanity intact when survival appears most in doubt.

And somehow, watching actual people - especially actors as finely tuned and copacetic as Mortensen and Smit-McPhee - slog through The Road's devastated vistas and heart-stopping scrapes looks less grim outside the confines of McCarthy's prose. If you haven't read the book, it's possible that you might assume that the love story between this father and son is embellished and played up, and that the movie's ending is a studio-inspired, tacked-on deus ex machina. While the movie actually has the arc, if not the tempo or trappings, of a Hollywood blockbuster, it is McCarthy's book in spirit, if not in letter. If it feels less grim in the end, it's no less haunting.