“Dark” is right. Christopher Nolan’s sequel to his Batman Begins is not only dark; it is searing and disturbing. The bad guys are very, very bad. These are not guys who do bad things because that is the only way for them to get what they want. These are bad guys who do bad things because they enjoy them. As the Joker (Heath Ledger, in his last completed performance) says early on, “That which does not defeat us makes us…stranger.”

But what is more unsettling about this ambitiously epic film is the way that it shows us how even the good guys are perilously close to being bad. We like duality in our superhero sagas, but we like the meek or ineffectual character with the hidden strength and ability — Clark Kent as the incorruptible Superman and Bruce Wayne as the eternally honorable Batman. But this movie is an exploration of the way that none of us, not even heroes, not even ourselves — none of us know exactly where our boundaries are drawn. Over and over in this film people find themselves crossing lines they once were certain that nothing could tempt or force them to breach, with the most fundamental elements of identity and integrity revealed as ephemeral.

In the last episode, we saw how billionaire Bruce Wayne, a damaged man, found his deepest essence expressed as a masked avenger, Batman. The pull of turning himself into a creature of the night to protect the innocent and put the guilty in jail was so powerful that he risked losing the woman he loved, his childhood sweetheart Rachel Dawes (Katie Holmes in the first film, Maggie Gyllenhaal in this one). But as this movie begins, the clean-up efforts by Batman and district attorney Harvey Dent have infuriated Gotham’s criminals, who are escalating their efforts and working together to spread corruption throughout the community so that no one trusts anyone. A man with a mask can be anyone — or more than one. Copycat Batmans (Batmen) are showing up with something the real Batman never carries — guns. “That wasn’t exactly what I had in mind when I said I wanted to inspire people,” Wayne says. The line between justice and vengeance is blurring.

Blurring of lines is one of the themes snaking through this film. Characters slide in and out, over and across lines of identity, principle, and purpose. This is a comic book movie and it has chases and crashes and fight scenes, including a astonishing somersaulting truck, but when it is over it is the wrenching choices, the internal confrontations, that reverberate. The most stunningly unforgettable moment concerns a choice made by a character who is on screen for less than five minutes. But because we know so little about him (far less than we think we know, as it turns out) and because the decision he must make is so heart-rending, his choice becomes ours.

And Batman’s time and place becomes ours, too. The setting is less stylized than previous Gothams, recognizably Chicago. This is a real city with windows opening up on sun light that is always on the other side of glass and steel. We, like the characters, are relegated to the shadows, the underground passages, the airless buildings, a kind of architectural mask.

The sense of dread, of corruption, of dissolution of structures permeates the film. A bad guy who is ruthless in pursuit of money or power is not nearly as scary and unsettling as one who cares about nothing — not even his own life — as long as he is messing with everyone’s head. Like the bad guy in “Saw,” the Joker likes to expose moral weakness and exploit hypocritical pretense to honor and integrity. “Some men aren’t looking for anything — just to watch the world burn,” says loyal retainer Alfred (Michael Caine). “They can’t be bullied, negotiated, or reasoned with.” And the greatest damage this kind of terrorism inflicts is that it no longer allows us to be the trusting, decent people we like to think we are.

Ledger, in his last completed performance, is mesmerizing. His tongue flicking like a lizard, there is a wetness to his speech that makes us feel as well as see the nerve-slashing wounds that give his face the grotesque rictus that imitates a smile. Instead of the careful clown-like make-up of previous Jokers, Ledger’s is smashed and smeared, chaos upon chaos. Bale continues to make Batman and Wayne compelling and Freeman and Michael Caine as Alfred are watchable as ever. “You complete me,” the Joker says to Batman. Ledger completes this film and his loss is just one more reason to walk out of it a little sad and dazed.