Why so serious? Because "The Dark Knight" wants desperately to be taken seriously.
It certainly takes itself seriously -- sometimes too seriously for its own good.
Even without the real-life death of Heath Ledger, who plays the movie's maniacal Joker, this sequel to 2005's "Batman Begins" cloaks itself in funereal black.
It's almost as if somebody sprinkled ashes in the popcorn.
Yet nobody's likely to choke; this is a movie perfectly pitched to the nasty desperation of the times.
Which means it's not much fun. Then again, nobody's in the mood for fun these days.
Except, of course, the Joker, who makes his own fun -- by triggering everyone else's fear.
And in Ledger, this "Batman" movie has a Joker for the ages. (Or, more precisely, the age.)
Unlike the jolly, look-at-me Jack attack Jack Nicholson unleashed in Tim Burton's 1989 "Batman," Ledger's Joker doesn't take over "The Dark Knight" so much as haunt it, establishing an anarchic spirit that permeates the proceedings.
You notice I haven't mentioned "The Dark Knight's" title character yet. That's because Batman himself (once again played by Christian Bale) often seems like a supporting character in his own movie.
And that's in part because the movie serves up such an overstuffed buffet of characters, plots and counterplots.
For starters, there's the Joker, fearlessly ripping off Gotham City's most intimidating criminals (led by a smilingly corrupt Eric Roberts), making life even more complicated for incorruptible Gotham City detective Jim Gordon (Gary Oldman) and his team.
And did we mention that Batman's vigilante presence has inspired a host of copycat caped crusaders who aren't quite as adept at vanquishing the bad guys?
Alas, writer-director Christopher Nolan and his co-writer, his brother Jonathan, don't bother to develop that potentially fascinating concept.
Instead, they turn their attention to crusading district attorney Harvey Dent (earnest Aaron Eckhart), a white-knight contrast to Batman's dark knight.
Dent's at the center of the movie's two interlocking triangles, providing a link between the Joker and Batman in one.
Because Dent's professional and personal partner happens to be none other than lovely lawyer Rachel Dawes (Maggie Gyllenhaal, much more convincing than Katie Holmes was in "Batman Begins"), they're also two sides of a romantic triangle that includes Rachel's ex-flame, billionaire-about-town Bruce Wayne -- alias the Dark Knight himself.
But let them weave their tangled webs and anguish over emotions and ethics. The Joker's got some bullet-spraying, bombs-away mayhem on his mind -- and whatever he thinks, he does without thinking.
Not without laughing, however. As one of his mayhem-on-wheels trucks reminds fellow Gothamites, "(S)laughter is the best medicine."
There's plenty of slaughter throughout "The Dark Knight," which packs plenty of punch in its spectacularly staged stunt sequences.
At least they seem to be spectacularly staged; with Nolan's slice-and-dice approach, you never can be sure, since much of the action is so choppy it verges on the incomprehensible. Sure, stuff blows up real good, but you're not always sure which stuff is blowing up, or where or why or how.
Sometimes Nolan's Cuisinart-style dramatic structure works; at other times, it drags out some peripheral sequences and truncates seemingly crucial scenes -- for example, a fateful, face-to-face confrontation between the Joker and Dent.
Which brings us to "The Dark Knight's" most successful face: its human one.
Veteran Oscar-winners Michael Caine and Morgan Freeman contribute welcome gravity and warmth to their limited but pivotal roles as Bruce Wayne's faithful family retainer Alfred and Wayne Enterprises right-hand man Lucius Fox, respectively.
Oldman and Eckhart, meanwhile, manage to make the good guys they play as interesting as they are admirable, always an impressive feat.
Just ask Christian Bale. He's one of the most intense, compelling actors around. (Check out "Rescue Dawn" or "3:10 to Yuma" if you need a reminder.) Bale's all sleek smirk as Bruce Wayne, but as Batman he can't always move beyond the cowl and the growl to tap into his character's divided soul.
That inevitably leaves the spotlight to Ledger, who responds with a performance that's both carefully controlled and gleefully unhinged, showing the Joker to be fully alive to the power of anarchy -- and his ability to create it by pushing people's buttons. Including ours.
Wisely, Ledger doesn't dwell on the "why" of this Joker's madness, leaving us to ponder that unanswerable question. (That's the tantalizing secret to all such memorably unsettling villains, from "Psycho's" Norman Bates to "Silence of the Lambs' " Hannibal Lecter to "No Country for Old Men's" Anton Chigurh -- no matter how many possible explanations we consider, we can never really be sure.)
It's a tribute to Ledger's talents, however, that we don't ponder them, or his tragic demise, while in the Joker's grip.
It's only after "The Dark Knight" fades to black (with the dutiful promise of another sequel, of course) that the sad truth hits us: The franchise may belong to Batman, but this installment belongs to the Joker.
Contact movie critic Carol Cling at firstname.lastname@example.org or 702-383-0272.