THERE ARE two incontestable facts about the members of the New York City police force:

1. They all have mustaches.

2. They all carry styrofoam coffee cups bearing the words, "It is our pleasure to serve you."

Beyond these meagre marks of consensus, the New York cops--and police in general--remain fundamentally ambiguous in the roles society assigns them and those they ultimately accept. Police are, theoretically, enforcers of laws designed by elected officials. In New York and much of urban America, however, where law-breaking is so widespread that cops decide not what to enforce but whether to enforce at all, police can become American justice. Do they want to be lawgivers, oracles of justice? Or do they want to go to work, come home (in one piece), raise a family and live like everyone else?

This is fertile territory for exploration and no recent effort in any medium is more ambitious than Prince of the City, which tells the story of a detective in a free-wheeling narcotics unit who decides to go undercover to battle against police corruption. From the day Danny Ciello (Treat Williams) decides to turn on his fellow officers and surreptitiously "wear a wire" (tape recorder), Prince takes a relentlessly compelling journey through the value system of the entire profession. But there is a fatal flaw that prevents the movie from capturing the meaning of Ciello's "turning." The character even says it himself early in the story, when confronted by two ambitious prosecutors who want to use Ciello to further their careers. "No one understands cops," Ciello screams. "No one understands us except our partners." And, alas, he is right. For all that Prince of the City pretends to unveil the world of the cop, each viewer can leave the film with his preconceptions about cops--whatever they are--entirely unaffected. In short, we don't know why Ciello does what he does, and that gap leaves Prince hollow at its core.

But if we accept Prince of the City as merely unravelling the ramifications of a police officer turning on his brethren, it fascinates absolutely, even over a corpulent three hours. Ciello is an informer, a curiously ambiguous role throughout American history, and director Sidney Lumet has accepted and used brilliantly that ambiguity, refusing to portray Ciello as either hero or villain. Americans have never been entirely comfortable with the informer, but informers have periodically (though often only temporarily) emerged as heroes. The name-namers of the McCarthy period offer the most striking example of evaporating national acclaim, while the Watergate tattlers have generally enhanced their reputations. Ciello falls somewhere in the middle.

Danny Ciello loves cops and hates cops; he wants to be buddies with everybody, but he is apalled at his colleagues' ways. He cares most about loyalty--loyalty to his partners and to the laws they enforce, though these are not always the law of the land. He also wants to save his own ass. It is an endlessly complex, perhaps even contradictory role, demanding an actor capable of expressing subtle character evolution in a man who is anything but subtle. Strangely, Williams vacillates from the superb--especially early in the final hour--to the abysmal--particularly in his first few scenes with the prosecutors. At times Ciello's self-acknowledged betrayal of his friends seems to be wearing him down to a single, raw and exposed nerve. Yet other times, Williams' posturing makes Ciello look ludicrous, like some hyperthyroid hampster racing his exercise wheel to no apparent result. A brilliant performance in the role of Danny might have clarified some of the mysteries about his motivations; a disastrous portrayal would make the film unwatchable. Williams strolls that spacious middle ground between epiphany and catastrophe.

Lumet decided to focus almost entirely on Ciello, ignoring most of the other questions this complicated film raises. New York City, for example, is almost entirely absent. Lumet shoots almost all scenes inside, presumably to show the profound isolation of Ciello's--or anyone else's--behavior. Lumet may have thought he already made his New York Movie in Dog Day Afternoon, with its wonderful characterization of the city's good-natured malevolence, or in Serpico, with its Good Guys vs. Bad Guys simplicity. One can only hope, by the way, that Lumet concentrated on interiors because he wanted to set a specific tone, not because he sought to solidify his status as the studio's favorite director--the man who always comes in under budget and ahead of schedule.

Lumet wisely does not go the ethnic cross-section route in selecting Ciello's fellow cops, avoiding the one Black, one Jew, one Irishman, one Italian and one WASP solidarity of scores of war movies. The New York Police force, its press releases notwithstanding, still operates in ethnic cliques, still works on a team concept. Lumet does not dress it up with any phony cross-cultural exchange. Even though more artistic exploration has focused on the criminal class than on its pursuers, anyone who has bothered to look at cops has seen the remarkable, almost symbiotic relationship among them.

Especially in a place like New York, the kinship is like nothing so much as soldiers on a front line. The horrible, griding adversity draws the men together as almost nothing else can, leading them to hate the system--both the crooks and the courts--that makes their world the way it is. So the cops live outside the system: sometimes they steal because they are greedy, sometimes because they figure they deserve the money more than some pusher. Sometimes they actively bust heads because they think it works as a deterrent, and sometimes they just ignore everything they see, waiting for the pension 20 years will bring them. Often the job, with its endless contradictions, proves too much. As Ciello points out, Mafiosi never commit suicide, while cops kill themselves in droves.

The very closeness of the relationship among partners in the face of such ambiguities presents the central theme of the movie. Lumet could have used the disintegration of that bond as an opening for examining the causes of Ciello's behavior. Danny Ciello somehow couldn't fit in. He loved the camaraderie, but something--just what is maddeningly unclear--made him rebel at his fellow detectives scorn for the system. At least partially driven by self-preservation and self-aggrandizement, Ciello is not noble, just understandable. He risks his job and his life to turn on his fellow officers, a move that Treat Williams sensitively portrays as impulsive, with Ciello unaware of his actions' real consequences.

The end product of Ciello's decision to turn provides the most illuminating moments in Prince of the City. In the end, Ciello inevitably reveals his own wrong-doing in the course of exposing crimes by his fellow-officers. A group of district attorneys must then decide whether to prosecute Ciello along with the other policemen. After Ciello has risked everything to place his trust in the judicial system for which his partners have lost all hope, the system turns on him.

The words spoken in the attorneys' graceful, paneled office come in a predictable stream of self-serving homilies. But the words are not the important part. It is the tone that matters: these people are in charge, they always will be, and they will care for their own kind. Cops, too, try to shelter their own, but their authority on the street is nothing in the courtroom. For a while, Danny Ciello does indeed seem to be the prince of the city, but in the end everyone realizes that the true monarchs would never yield their power to a cop.