Sequels are inherently difficult. A pre-primed audience arrives on a wave of anticipation created by the original film that was good enough to command a second bite at the box-office apple.

That's the dilemma Christopher Nolan faced as he prepared to make "The Dark Knight." And he faced an additional obstacle of his own making. His 2005 "Batman Begins" was both a hit with the fans and redefined the Batman franchise.

Nolan went as far afield as he could from the campy television series of the 1960s. He even managed to erase the memories of failed sequels to the previous "Batman" movies, more remembered for plastic nipples on the costume than plot.

"Batman Begins" introduced the dark side of the Caped Crusader. And audiences responded. Now all Nolan had to do was make a bigger and better movie as a sequel.

"The Dark Knight" is that in every way, rising above the first on the wings of its dark creature of the night.

The film picks up the action a few years later. The crime rate in Gotham City has dropped because of the presence of Batman (Christian Bale). Prosperity can breed discontent. There are some who are still a little uneasy with this vigilante style of justice.

In this dark time emerges a savior: District Attorney Harvey Dent (Aaron Eckhart). He is fighting his own crusade against crime, but in the light of day. Naturally, the criminal element does not appreciate his efforts. The most heinous of these criminals is The Joker (Heath Ledger). Just his appearance - hastily applied clown makeup to barely cover scars that extend from the corners of his mouth to his cheek bones - is enough to send shivers up the spine.

In a tour de force performance, Ledger plays The Joker as the ultimate villain. His vile actions are not motivated by greed, politics or fame. He is evil for evil's sake.

The battle between Batman and The Joker gets even more complicated when an accident transforms white-knight Dent into the revenge-minded Two-Face.

Nolan loads the movie with enough sequences to feed the addiction of any action-film junkie. The battle on the streets of Gotham City, which includes a garbage truck, semi and cool Batman vehicles, alone is enough to win fans' hearts.

At times, the film seems to have slipped past its PG-13 rating into more restricted areas. On close examination, Nolan has gone the way of Alfred Hitchcock in that scenes are shot in such a way as to make the viewer think there is more violence than is really being shown onscreen.

As with Batman himself, there is much more going on behind the mask of action. This is a movie that looks at the heart of good and evil, and the line between the two is as delicate as the line between life and death. The movie demands post-screening discussions of the cool stunts, the deep writing or both.

Under other circumstances Bale would be getting all the praise for managing to make Batman such a serious character despite all the comic-book trappings. But Ledger's performance outshines all others. This is said not in sentimental reaction to Ledger's death in January, after the film had finished shooting. Ledger makes an acting transformation that is complete.

It is both a brilliant and sad reminder of the work he could have done.

All of which goes to show that a sequel does not have to be a letdown. "The Dark Knight" is a triumph from beginning to end.