It's been a joy to follow The Dark Knight director Christopher Nolan's career through his last five movies, beginning with Memento. His films have now consistently explored the thin line that separates the civilized from the primal, obsessive survivalist. Usually, they focus on a protagonist who finds himself amidst moral issues, but in The Dark Knight, the sequel to his Batman Begins, no less than civilization itself finds its soul and humanity being tested.

Batman's home turf of Gotham City has proven to be a fertile ground for Nolan's metaphors on the human condition as it exists within the state of today's world. In Batman Begins, he touched upon it as a collapsing city, burdened with class divisions and increasing crime, and corrupt enough for a villain mastermind to justify its being wiped out. But in that movie, the city played a passive role, its existence being fought for by, essentially, god-like figures, one of whom is benevolent, i.e. Batman.

The Dark Knight ups the ante by having Gotham City be directly responsible for the directions it takes and the decisions its citizens make. As such, it stands in for any of today's civilized nations and becomes ripe for thematic exploration. And Nolan wastes no chance to utilize this world for a tale about how fragile our morality is. Overridden with criminals, Gotham City is in a constant state of panic, with Batman (Christian Bale) being one of its only beacons of hope -- but even his presence is confusing, in that his motives appear ambiguous to the public. Enter a brash, effective District Attorney, Harvey Dent (Aaron Eckhart), providing a new light for the city, relentlessly prosecuting all captured criminals in the face of threats to his life. Still, the thread of hope he represents feels like a constant struggle against overwhelming obstacles.

Nolan then introduces The Joker (the late Heath Ledger, whose potential was, alas, just starting to blossom) as the agent of chaos. He acts as a perfect device to rattle the nerves of a metropolis's frayed citizens, as the character doesn't require a traditional motive -- money, power, politics. He shakes things up just to do it -- we're not even sure if he gets any real kick out of it -- but it makes him all the more dangerous. He's there to be the shapeless enemy, a force of destruction that strikes with no warning or cause -- a pure terrorist. And his schemes are enough to put a city's humanity to the test.

The Dark Knight spotlights the real-world theory that, when ordinary people are faced with danger to themselves and, perhaps more so, their loved ones, they are ready to sacrifice anything in the name of security and protection. A mundane foray into this theme might have proven debilitating, but the movie is able to make the most of delving into it for two reasons. First, The Dark Knight is a crackling crime thriller -- not a typical superhero movie, it has more in common with gangster films featuring good, incorruptible cops facing off against merciless, murderous crime lords. Despite being about a vigilante in a bat suit, the story feels real, and its depth is increased by featuring a truly scary villain. Ledger's Joker is frightening enough to generate actual, palpable suspense. As a thriller, the movie comes up aces.

Second, the film is not content to merely preach the line about losing faith in people once everyone determines things are bad enough to declare, "every man for himself." There is a true, optimistic belief in the best of people, and the movie is even willing to press that to a breaking point in a climax that doesn't even directly involve Batman. Not here to ask where morality comes from, The Dark Knight mainly recognizes that it is there, and there is reason to have faith in it.

Nolan's movie contains so much that's relevant to today's political climate, and it doesn't let the answers come easy from any direction. More than once is murder offered as an option to protect/avenge loved ones. A S.W.A.T. team is on the verge of killing innocent people merely because they don't bother to check more carefully during a hurried, desperate operation. Batman himself resorts to physical threats to obtain information, and even implements a secret high-tech analog of The Patriot Act in order to locate his main enemies. But he also refuses to commit willful murder (even after losing loved ones), and seeks to return to his ethics even after his enemy dictates that ethics have no grounding in this world. Both sides are separated by a thin line, and Nolan is always willing to let his characters cross it, one way and the other, to show us just how challenging it is not to be human, but to be humane.

(Released by Warner Bros. and rated "R" for intense sequences of violence and some menace.)

Review also posted at www.windowtothemovies.com