By now, you’d think Sidney Lumet wouldn’t have it in him to make another police-corruption drama. This is, after all, the director who gave us Serpico and Prince of the City. The latter, made in 1981, was such a densely packed maze it seemed that Lumet had all but exhausted the dramatic possibilities of coverup and betrayal.

At first, his superb new movie, Q≈A, seems like more of the same. Set mostly in Manhattan — or, more precisely, Lumet‘s Manhattan, that harshly mesmerizing, fluorescent — lit urban purgatory — the film is about a dedicated young assistant DA, Al Reilly (Timothy Hutton), who is called on to investigate a case of ”justifiable” homicide. The figure under investigation is Lt. Mike Brennan (Nick Nolte), a veteran cop who’s a legend throughout the NYPD, both for his kick-ass bravado and his fanatic loyalty.

It seems that during a (supposedly) routine investigation Brennan blew out the brains of a Hispanic drug dealer. He claims it was self-defense, but we’re shown otherwise. The murder is the film’s opening scene, and it’s made clear Brennan executed the guy in cold blood.

By revealing Brennan’s guilt from the get-go, Lumet both dampens and heightens our suspenseful expectations. Q&A isn’t merely about the mechanics of a cover-up (which it pretty much takes for granted). It’s a tale of overlapping conspiracies in which personal and professional corruption have merged in subtle, sometimes shocking ways. Having dug through this terrain twice before, Lumet has reached a new, visionary pitch of despair and searing cynicism. Q&A is his darkest, most labyrinthine drama yet. The movie is an epic portrait of an urban-bureaucratic nightmare — it’s about a criminal-justice system so saturated with cronyism and rancor it’s beginning to strangle itself.

Eager to wrap up his first big case, Reilly begins to interview witnesses, and a few of them poke holes in Brennan’s story. Still, the most suspicious person in the case is Brennananimself. A bluff, towering Irish-American, this cop’s cop is a self-appointed king of the streets, a racist and ”fag killer” who leers with pleasure when he talks of beating the crap out of some poor black or Hispanic ”animal.” More than just arrogant, he’s a monster — a proudly hateful, street-punk megalomaniac. Though he claims to have killed the drug dealer in self-defense, his grin tells you he would just as soon have done it for fun.

Slowly, Reilly’s investigation unearths a rat’s nest of disreputable goings-on. I don’t want to reveal too much (part of the pleasure is seeing the connections slowly lock into place), but the story takes Lumet into bracing new areas of corruption and underworld camaraderie. There are scenes with a newly businesslike Mob elite, in which the order to execute someone is handed down with a chillingly offhand, almost corporate hauteur. And there are excursions into the glamorously morose New York demimonde of transsexuals and drag queens. The principal witness in the case is a sultry homosexual stage performer (played by the terrific Paul Calderon) with mysterious connections to Brennan.

Stylistically, the movie is pure Lumet: two-and-a-quarter hours of gritty, in-your-face realism. Still, Q&A doesn’t have the muckraking drive that marked Serpico and, to a lesser extent, Prince of the City. The movie is steady, deliberate, and a little detached. It’s a meta-mystery, like Chinatown, with a narrative so intricate the cop movie as chess game — it suggests a Kafka-esque vision.

Following the plot takes a bit of work (the key is paying attention to the names), yet it all adds up, and Lumet, adapting a novel by onetime New York assistant DA (and now justice) Edwin Torres, has a bold new theme. Q&A is about the disastrously entrenched tribal racism of New York cops, criminals, prosecutors, judges-an ethnic hyper-awareness that has become a rotting spiral of violence, complacency, and dread.

Never in a movie have the racial epithets flown as thickly and ripely as they do here. Lumet did the script himself, and the dialogue often sounds like a David Mamet play without the comic preciousness. For these cops, racial divisions aren’t a matter of prejudice but a primal reality — the ultimate test of whom you can trust. An open bigot is considered far more honorable than a bogus liberal. Lumet pushes the moral ugliness as far as he can. His vision may be something of an exaggeration, yet we can’t hide from the essential truth of it, any more than we could hide from the truths of Do the Right Thing.

The acting is superb throughout. Armand Assante, who till now has seemed something of a lightweight, comes through with a major performance as Bobby Texador, a strutting Hispanic drug kingpin fighting to go straight. Assante holds the screen with his hypnotic vocal rhythms (he alternates angry staccato bursts with seductive silences), and his hand gestures are poetry; he’s both magnetic and moving. And Nolte simply outdoes himself. As the treacherous Brennan, a man of many more layers than he lets on, he gives a performance of venomous brilliance — it’s the most daring, lived-in portrait of human evil since Dennis Hopper in Blue Velvet.

Q&A has its flaws. As a hero, Timothy Hutton’s Reilly lacks resonance, and a subplot in which he struggles to reconcile with his former fiancee (played by the director’s daughter, Jenny Lumet) feels schematic and phony. The movie is trying too hard to show us that the metaphysical seed of racism exists even within this idealistic fellow. And during the last third of the picture, when the action moves to San Juan, the pace needs to build more than it does; Lumet just keeps spinning out scenes in his earnest, determined way. Still, Q&A is a major film by one of our finest mainstream directors. As both a portrait of modern-day corruption and an act of sheer storytelling bravura, it is not to be missed.