The Road can be a tough slog, but the journey is a rewarding one.

Director John Hillcoat's film is true to the spirit of Cormac McCarthy's Pulitzer Prize-winning novel - if anything too much so. Fans of the book who worried that Hillcoat might sugarcoat it, going with a Hollywood ending and thereby softening the novel's impact, will be relieved.

Relief may not be the feeling those unfamiliar with the book will have, however. The story, set in the decaying aftermath of an unnamed disaster that has wiped out much of the population and all of the rules of law, society and even humanity, is unquestionably grim.

Stick with it, though, and you'll find the promise of hope among a ruined world. If everything you know and love is gone, can one ask for anything more?

Viggo Mortensen plays the unnamed man at the story's center. Sick and exhausted, he is trudging his way through a frozen wasteland. He is trying to survive, but his main purpose is to protect his son (Kodi Smit-McPhee), his "world entire."

There is much from which to protect him. Society in any meaningful form is shattered. Anyone else they run across is likely to be a robber, if not a cannibal. It's every man for himself in the extreme. Whatever happened to the world has left it dark, cold, ugly and dangerous.

This complicates moral choices. Is anything off-limits when it comes to survival? The man, as far as we can tell, is a decent sort. Yet in the world we know, some of the choices he makes, the actions he takes, would be unthinkable if someone described them.

But this is not the world we know. It's the world they know, fractured, corrupted, ruined. That changes things.

We learn of the man's former life through flashbacks. He and his wife (Charlize Theron) were happy. But then the disaster, whatever it was, took place, leaving the world without power, without communication. A bad situation gradually gets worse as society dissolves. The man is convinced that it will be warmer at the coast, that other survivors will be headed there, too. His wife thinks survival is impossible, something that will affect her own decisions greatly.

If The Road were a relentless journey of despair, it would be too much. But Hillcoat offers relief in the occasional small victory - an unopened can of Coke, for instance, a treat the boy has never tasted. Or a stash of fresh supplies. And slowly, the boy's innate sense of decency even begins to thaw out his father a little.

Not too much, though. That could prove fatal. Nothing must prevent the boy's survival.

Mortensen is typically intense; if anything, more so than usual. Smit-McPhee is quite good, looking for some sense of a child's happiness in what seems a hopeless existence.

But The Road is not just a story of survival or of man's place in a universe gone so incredibly wrong. Above all, it is a tale of a father's unconditional love for his son and how that love makes life worth living, ferociously.

Whatever it takes.

Rating: ****