The title adjective is certainly well-chosen in Christopher Nolan’s sequel to “Batman Begins,” his 2005 resuscitation of the DC Comics franchise that had earlier been wrecked by Joel Schumacher’s Las Vegas-style razzmatazz. “The Dark Knight” is one grim and downbeat chapter in the saga of Gotham’s Caped Crusader.

That’s true not only visually—Wally Pfister’s splendidly-judged cinematography gives the images a burnished gray-and-silver sheen, especially appropriate in a movie set mostly at night—but in terms of the themes in the screenplay Nolan wrote with his brother Jonathan. They concentrate on the thin line separating good and evil, light and dark. That’s the thrust of the central battle between billionaire Bruce Wayne (Christian Bale), the ever-more reluctant vigilante trying, in his outlaw fashion, to save the city from its crime lords (while copycats try to follow in his footsteps), and the Joker (Heath Ledger), the war-painted, anarchistic, totally amoral psychopath who embarks on a campaign of mindless destruction and death not so much—as he initially claims—to unmask Batman, but to force his mirror image (“the good freak,” as he puts it) to the dark side too. It’s also the arc in the subsidiary plot centering on crusading D.A. Harvey Dent (Aaron Eckhart), the new romantic interest for Bruce’s true love Rachel Dawes (Maggie Gyllenhaal, replacing Katie Holmes), a man of integrity and dedication whose altruism is tested to the max by the Joker’s machinations. It’s around these four that the other characters circle—Gary Oldman’s straight-arrow Lieutenant (later Commissioner) Gordon, Michael Caine’s avuncular butler Alfred, Morgan Freeman’s smoothly efficient master of weapons technology, Eric Roberts’ nasty mob boss, and a host of others.

They’re all cogs in what might be, without the cowls and greasepaint, another episode of “Mission: Impossible”—not quite a superhero movie (Batman has no special powers, after all), but one about an extraordinary human crime-fighter. And like the first installment in Nolan’s series, it’s mostly a smooth and accomplished piece of filmmaking, not least for Pfister’s sterling images (caught by this writer in their partial Imax form, with six big sequences shot for the oversized screen while the rest of the picture remain in standard widescreen format) and some impressive set-pieces, like an aerial abduction in Hong Kong and a duel between the Batmobile (and later the Batcycle) and an eighteen-wheeler in the city streets, carried off in a way that downplays the CGI pizzazz in favor of a more realistic look. Best of all, it improves on its predecessor by refusing to lead up to one of the splashily interminable finales that most directors feel compelled to feed to audiences in pictures like this nowadays. It ends more quietly and ambiguously, in a spirit of pained resignation rather than simplistic triumph. The only comparable example is in the latest incarnation of the other iconic DC character, “Superman Returns.”

But there are drawbacks. One is that while the bigger action moments are well-choreographed, the smaller ones—the fight scenes in which Batman dispatches one foe after another—are messy, shot too close-in and edited so that it’s impossible to see the moves. (That was a problem in the first installment, too—which suggests that it’s a deliberate choice, designed to keep Batman’s maneuvers mysterious. But it was a bad one then, and it’s a bad one now.) Another is that for all the crispness of the writing (there are quite a few nice one-liners), the plot convolutions don’t quite click into place with ideal precision. Take just one example: at a point late in the film, we’re apparently meant to believe that Joker has cannily manipulated things to put himself in a position to further his plans though it appears he’s lost. If you try to parse out what’s preceded, however, it’s an awful stretch. Or again, there’s a change of character late in the game that’s necessary to resolve the dichotomies the plot has set up; but it’s handled too brusquely to give the last scenes the resonance they’re meant to have.

You’ll be inclined to let such problems pass, however, because in almost every case they give Ledger the opportunity to let loose, and his high-risk performance is easily the picture’s most exciting element, overshadowing even the effects. Saying so isn’t a pro-forma nod to an actor who died far too young; it’s simple fact. Ledger had been building an impressive resume before this role—almost unrecognizable but unnervingly convincing as an aging surfer-dude (“Lords of Dogtown”), heartbreaking in “Brokeback Mountain,” and, for a change of pace, raffish and charming in “Casanova”—and this is another remarkable turn. Ledger’s Joker is no comic figure in the Jack Nicholson mode; he’s a genuine madman driven to sow chaos, and Ledger plays his inexplicable malevolence to the hilt in a daring display that, had it gone wrong, could have been disastrous. But Ledger knows how to take the mania to the edge without toppling over.

Unfortunately, beside such a display Bale comes across as rather pallid. He’s a great young actor, too—“The Machinist” and “Rescue Dawn” certainly proved that. And he manages some good moments as the supremely self-confident Wayne, especially in his scenes with the effortlessly skilled Caine and Freeman, and to a lesser extent with Oldman, who brings surprising depth to the principled Gordon. But once in the suit, Bale goes stiff and rather dull, emitting a sepulchral, mechanically enhanced voice that quickly becomes monotonous. Frankly it’s not a part that brings out the best in an actor—it’s mostly chin, after all—and Bale suffers from having much more to give than it asks of him. Gyllenhaal is an improvement on Holmes, but Rachel remains a bland, featureless character, and she can’t do much to enliven her. By contrast Eckhart brings considerable energy to Dent, though he’d have needed more screen time to flesh out the troubled D.A. Roberts, meanwhile, is so convincingly sleazy that you’d swear it was second nature to him. Lesser roles are for the most part adequately filled—Anthony Michael Hall, of all people, plays a television anchorman too anxious for scoops—though the people involved on two ferries toward the close (a sequence that aims for far greater punch than it actually has) seem to be of the Jon Lovitz school of “acting!”

While the physical production is aces, the music score, from the combo of Hans Zimmer and James Newton Howard, is curiously generic. But the Chicago locations, standing in for Gotham, are very well used.

So while one can point to some weaknesses, “The Dark Knight” is overall a superior sequel, remarkable for its blend of serious themes and crowd-pleasing action—and with a performance by Heath Ledger that’s a memorable coda to a career cut far too short.