"ALL IT TAKES is one bad day to reduce the sanest man alive to lunacy," says the Joker in Batman: The Killing Joke. Written by Alan Moore, illustrated by Brian Bolland, and published in 1988, The Killing Joke is a landmark story from Batman's 70-year comics history—and the definitive story about the Joker, a sadistic sociopath in clown make-up and Batman's most infamous and fitting nemesis. "That's how far the world is from where I am," the Joker continues, lecturing Batman in a tone somewhere between a confession and a warning. "Just one bad day."

Heath Ledger died in January, and it was an ignoble, awkward death. It was also a blow: One of the few legitimately great young actors, Ledger had managed to snag an Oscar nomination for Brokeback Mountain just six years after appearing in stuff like 10 Things I Hate about You, and even in roles that could've been utterly pedestraian—I first remember being impressed with him in the otherwise forgettable Lords of Dogtown—he brought sharp instincts and unfailing charm to his performances. Especially in histrionic Hollywood, there's a tendency to go overboard with the eulogizing when somebody dies, but the outpouring of grief after Ledger's bizarre, unexpected death was, I think, deserved: Ledger was good, and he was going to get better, and he was gone.

The fact that Ledger's final completed role is that of the Joker in The Dark Knight, Christopher Nolan's eagerly anticipated sequel to Batman Begins, is, to say the least, disconcerting. But all the same—three years after Batman Begins, and seven months after Ledger's body was found—The Dark Knight is all the things audiences are hoping it will be. It is bold, bombastic, and badass. There are sublimely orchestrated action sequences, stunningly gorgeous cityscapes, and elegantly conceived bank heists and abductions and interrogations. Nolan, after dipping his toe into the unpredictable waters of big-budget filmmaking with Batman Begins, is now 10 times more confident and daring: The Dark Knight, shot in icy blues and rich blacks, feels as much like a smart, stylized Michael Mann crime epic as it does a summer blockbuster. It's all the better for it, too, especially coming in the midst of a summer heavy with superhero fare: We've already had Iron Man, The Incredible Hulk, Hellboy II, Hancock, and probably 10 or so others I'm forgetting.

The Dark Knight is the most thematically dense of these films, but it's also the most convoluted: Even at two and a half hours, the film crams in a whole lot, probably more than it should. The broad strokes: Smirking, debonair billionaire playboy Bruce Wayne (Christian Bale) still dresses up like a bat to fight crime (in the process forcing his otherwise smooth, sophisticated voice into a distractingly guttural growl that can best be described as belonging to a grumpy Muppet). Things get significantly trickier when the vicious Joker violently sweeps through Gotham, plunging the city into paranoia and anarchy. Meanwhile, Wayne continues his efforts to get a piece from Rachel Dawes (here played by Maggie Gyllenhaal, a welcome replacement for Batman Begins' Katie Holmes), who happens to be seeing Harvey Dent (Aaron Eckhart), Gotham's charming, dedicated district attorney. "Gotham needs a hero with a face," Wayne says of Dent (FORESHADOWING ALERT!), and soon, the fates of these characters—along with that of Lieutenant Gordon (Gary Oldman), the good cop to Batman's bad cop—are bound together. It's also worth noting that a lot of things blow up and that Batman rides a fucking sweet motorcycle.

The Dark Knight's screenplay is full of fantastic characters and engaging ideas and visceral sequences, and one senses Nolan trying to cram all of them in, twisting each one a bit to make it fit into the whole. It works, usually, though there are occasional scenes that don't, and a few plot points and characters that frustratingly fade away. Whatever annoyances pop up, though, seem minor compared to the whole, which is impressively robust in scope and tone.

Perhaps the most notable thing about The Dark Knight's tone is that it's so relentlessly and unapologetically... well, dark. The Dark Knight is fun, but there's also a stark, twisting anger to it, a sinister, cynical, nihilistic edge that can't be denied. Part of this is by design—the tense, simmering script, by Christopher Nolan and his brother Jonathan, focuses less on Batman and more on his foes—but the darkness is also inseparable from Ledger, whose death has colored the film in ways that are impossible to shake.

Which brings us back to Ledger's turn as the Joker—a performance that's making a whole lot of excitable critics point out things like how the last time a posthumous Oscar was awarded was for Peter Finch in 1976's Network. Regardless of any posthumous awards, it's Ledger's strange, fascinating work in The Dark Knight that'll be remembered. To say that this is one of Ledger's best performances—if not, in fact, his best—is the easy way of putting it. Trickier is noting all of the ways in which his funny, terrifying Joker is so affecting: Slouching his head forward and nervously smacking his lips, he shuffles and darts about, speaking with a lilting, excitable, unpredictable cadence that imbues every line with hints of a rage festering behind his scarred face. "You have all these rules, and you think they'll save you," he taunts at one point—and whether he's brandishing a nasty-looking knife or merely his wits, the Joker's malevolence, insanity, and humor never cease to be riveting. It says a lot that in a film crammed with fine performances from Bale, Gyllenhaal, Eckhart, and Oldman, The Dark Knight still loses a good part of its punch whenever Ledger's not onscreen.

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Back to that comic book: A few panels later into The Killing Joke, the Joker keeps up his contemptuous harangue: "You had a bad day once, am I right?" he asks Batman. "You had a bad day and it drove you as crazy as everybody else... only you won't admit it! You have to keep pretending that life makes sense, that there's some point to all this struggling." There's a beat, here, and the Joker's next line gets its own panel: "God, you make me want to puke."

Cheery stuff, that, but it's also pretty great—and, it turns out, pretty influential. With its roots in comics like The Killing Joke, The Dark Knight is the latest in what's shaping up to be an increasingly unique and impressive series of Bat-movies. That the film is at once dependent on and a sort of memorial for Ledger is prickly and discomfiting—that a promising young actor's career would abruptly end with him in a summer popcorn flick, playing a sadistic sociopath in clown make-up, feels like the painful punch line of a weird, unfunny joke. But then again: The fact that The Dark Knight is so strong, in large part thanks to Ledger's performance, somehow makes that punch line feel oddly, hauntingly appropriate.