In one of the key scenes in The Dark Knight, the film's major villain, The Joker (played by the late Heath Ledger), is seen berating what he sees as society's over-reliance on rules and norms. 'I'm the agent of chaos,' he purrs in a mixture of comical hilarity and spine-tingling menace.

The Joker does much more than instigate brutal robberies (as seen in The Dark Knight's staggering opening sequence) or inspire pandemonium among the general population of Gotham City. It's his ability to stir up the dark side of nearly everyone in the film - from seemingly incorruptible public crusaders to the city's ordinary denizens - that drives Christopher Nolan's relentlessly pulse-racing vehicle.

It's more than apt that this is the first Batman film in which the character's name doesn't appear in the title - while still remaining central to the narrative. The labours of the rubber-clad icon are almost eclipsed by the message that runs through the film: What does it take to convert normally civilised humans into crazed lynch mobs, and change white knights into shady monsters?

The Dark Knight is much more visually ravishing and adrenalin-inducing than Batman Begins (Nolan's first outing in the franchise). There is no shortage of high-octane action scenes, deafening explosions and car-flips augmented, of course, by the Batmobile.

This being a Nolan film, however, introspection is never far away. It's not just Batman/Bruce Wayne (Christian Bale) who's struggling with his conscience, as he sees his work - clearing Gotham of its swaggering criminal syndicates - upsetting the equilibrium built upon the uneasy collaboration between law enforcement agencies and the mob.

The other part of the equation is the emergence of Harvey Dent (Aaron Eckhart), the city's young attorney general, who brooks no deals with the underworld as he sets out to rid the city of corruption and crime. This defiance will later be put to the test as another facet of his personality rears its ugly head.

Much emphasis has been placed on the film's technical aspects - the six major action sequences, one of which involves Hong Kong's skyline, are all shot on Imax cameras, and they certainly provide startling visuals. But it's as a study of the characters - or, to an extent, morality in general - that The Dark Knight emerges as one of the most impressive and, paradoxical as it might sound, reflective summer blockbusters to appear in recent years.

In what would be his last role before his death early this year, Ledger is at his best. He is menacing throughout, even when dressed in the most hilarious attire or uttering to Batman, 'You complete me' - a phrase borrowed from a sappy scene in Jerry Maguire.

What lingers beyond the film's ending credits, however, is Nolan's questioning of heroism. The Dark Knight provides much more than just ear-splitting entertainment.

The Dark Knight opens today