Watch a recap of the latest season of The Wire

Tom Waits sings: “You gotta keep the devil way down in the hole.” But how? And who and where is the devil, anyway? That, in a nutshell, is the modern American television cop show. The world has gone wrong. The devil’s out of his hole. How do we get him back down there? There is no easy answer; probably, there is no answer at all. But there is one big consolation. The cop show is in the midst of a new golden age. With shows such as The Wire, The Shield and Dexter, the Americans are demonstra-ting, once again, that they are uniquely capable of making and sustaining the best television in the world.

The Waits song is the theme tune of The Wire, the most radical of all the new-wave cop shows. The fifth and final season is just about to start on FX. Watch it - but I warn you, it is hard work. There are few concessions made to normal narrative expectations. The language is hard, and stories are often buried beneath random events. Yet, after a time, you notice things.

For example, I felt a distinct prickling in my scalp while watching a scene in which two young black drug dealers in Baltimore are sitting at a chessboard. Their boss comes over and points out that one of their moves is illegal. They respond that they are not playing chess, they are using the pieces to play checkers (draughts). Appalled, the boss starts teaching them the correct chess moves and the names of the pieces. The “little bald bitches”, he explains, are the pawns. As he talks, the hierarchy of the chessboard becomes the hierarchy of their lives, and the kids who don’t know how to play are, of course, the little bald bitches.

Or there is the moment when another young hoodlum turns on the television to see a sleazy white guy making his pitch for mayor of Baltimore. He flips it off without thinking and launches himself into the midst of a very violent, shoot-’em-up video game. It is simple, but, in context, it makes you gasp. This is writing of the highest quality.

Neither The Shield nor Dexter is anything like as demanding as The Wire - which is why they get awards and much bigger ratings. Both are recognisably conventional in form and both, in different ways, lead the audience by the hand up to their dramatic climaxes. The Shield, however, shares The Wire’s harsh and dirty realism. Dexter, in contrast, is glossy melodrama, bloody grand guignol for technophile postmoderns. But all ask one big question: who and where is the devil in this jagged, lethal landscape? And, in each case, there is absolutely no prospect of getting him back in the hole. This isn’t a war, as one cop says in The Wire, because “wars end”.

The detective story, in all its various forms, works because it is always based on the same obvious but undeniably true metaphor - searching for the villain is the correlative of everybody’s search for meaning, purpose and identity. Usually, though not always, the search, successful or not, is intrinsically virtuous. The greatest of all fictional detectives, Raymond Chandler’s Philip Marlowe, is a mess, and, frequently, incompetent, but he is morally driven and, in the end, always revealed as better than the world through which he walks. His interior monologue is a masterpiece, Hamlet made modern.

Yet when we get to the postmodern, we find ourselves denied the consolations of Marlowe. There are two aspects to this. The first is moral. The idea of the virtuous search and the honourable detective no longer seem credible. The Wire is suffused with a dark vision of the moral equivalence between the drug dealers and killers and the formally law-abiding members of Baltimore’s hierarchy. In The Shield, we are constantly offered heroes, but, just as immediately, they are snatched away by revelations of their corruption and thuggery. Dexter takes this to the melodramatic limit. Our hero in the Miami police forensics department is a “blood spatter” expert. He can tell what happened by the shapes and patterns made by flying blood. But he is also, in his me-time, a serial killer. He “Takes Life. Seriously” runs the brilliant slogan for the show. So he is a bad guy, then? Well, no, because he only kills bad guys the cops aren’t going to catch. He enjoys killing, but he keeps it morally cleanish.

The second, related aspect of the postmodern cop show is the death of the real. Marlowe, like Sherlock Holmes before him, confronts a baffling world, but, always, it makes sense in the end. This tradition is maintained by current conventional cop shows such as Law & Order and CSI. In the first, the legal process is the solidity that anchors the entire equation. However slippery a case, the law is real. In CSI, the glamorisation of forensic science is founded on the faith that here, at least - in the apparent objectivity of the technology - lies the truth.

But in this postmodern trilogy, all such certainties have gone. If mayoral candidates behave like drug lords (The Wire), if the cops who know and seem to care are brutal and corrupt (The Shield), or if the forensics guy takes a little too much pleasure in his work (Dexter), then it is not just morality that is being subverted, it is reality. What is the truth? There is no way of knowing. Vic Mackey, the hero/monster of The Shield, is the embodiment of this. Cunningly, the show’s creator, Shawn Ryan, gives Mackey every heroic attribute - he is a well-meaning if flawed family man, the moral compromises he makes all seem in the service of a higher good and he is ferociously loyal to his men. But he is a thief and a killer and, as everybody around him realises, he is at least as much of a problem as a solution.

In this shifting moral and epistemological miasma, old forms persist as pastiche. There is a sublimely ridiculous scene in The Wire in which the drug gang holds a formal meeting - “The chair recognises. . . ” And in Dexter, Marlowe’s interior monologue is adapted to become the eerie, emotionally disconnected musings of a mentally crippled serial killer torn between blood lust and his father’s desperate injunction that he should only kill people who really deserve it.

Dexter is Marlowe gone horribly wrong, just as the gangsters form themselves into a distorting mirror of city hall.

Which brings me to the real hero of all these shows: the city. The Wire is most explicit about its true hero. “The Wire is not interested in good v evil,” says its creator, David Simon. “We are bored with the theme. The Wire is about the American city: how it works or doesn’t, and, ultimately, what is at stake for all of us in these times.”

I asked the sociologist Sudhir Venkatesh, who spent years among Chicago drug gangs, about the current interest in gang culture. “The same reason,” he replied, “we’re getting Barack Obama. It comes from the exhaustion of received political wisdom and social commentary. The world is changing, and old categories don’t work. The nation state is an anachronism; we are global creatures. The Wire shows the last desperate acts of an American city - I think the American city is growing somewhat irrelevant and needs to rethink its role.”

Each of The Wire’s seasons is centred on a different city sector - the cops, city hall, the gangs, the dock workers, the teachers and the media. All become crushed into the same nexus of eternal conflict and failure. In fact, Venkatesh found that, realistic though the show is, this flattening is the one thing Simon may have got wrong. He watched the show with gang leaders. They liked it, but said they could tell it was written by whites. Everything was treated as a game in which all were equal players. But, in the real world, there are winners and losers, and the blacks are always the losers. On the other hand, perhaps Simon is one step ahead of this - he certainly shows the black-white division as incurable and effectively eternal. But, in Baltimore, this is as much as a problem for the whites.

“Tomorrow morning, I’ll wake up white in a city that ain’t,” says Carcetti, the mayoral candidate, with a despairing shrug.

The Shield is set in a fictional district of Los Angeles called Farmington. A special police station has been established in a disused church (an irony too far for my liking). Farmington is as bad as it gets in the inner city. Drugs wash through the place like water, and only Vic Mackey and his “strike team” seem to have any grasp of the competing gangs and nationalities involved. Constantly, Vic and his boys are kicking down the door of a cheap wooden house to find yet another indecipherable scene of slaughter. Meanwhile, we glimpse the squalor of local government, as the station’s captain climbs the unimaginably greasy pole of LA politics.

Dexter is set in Miami, a suitably floating world of more concealed violence. The place is a pleasure zone in which everybody’s psychosis is hidden behind a mask of easy fun. In the opening-credits sequence, Dexter himself - brilliantly played by Michael C Hall from Six Feet Under - is shown breakfasting, flossing and putting on the precisely smart-casual clothes of the moment. He wears the good-life mask like everybody else, but, for him, it is no fun, it is just the necessary mask for his true identity as a killer of unimaginable coldness. Dexter is the supreme urban dandy - “My instincts are impeccable” - but his very existence makes it clear that Miami is just as disordered as LA or Baltimore. It just hides it better.

The Wire and Dexter are products of the subscription channels HBO and Showtime, respectively (The Shield is from Fox/Sony). Their sheer quality, combined with that of previous series such as The Sopranos, is rapidly making HBO, in particular, into America’s national theatre. Freed from the demands of advertising and the moral and marketing qualms of the networks, content is once again king, and the result is some of the best storytelling of our time.

You may not like what the stories tell and, indeed, sometimes I find the relentless moral equivalence of The Wire a little too easy, a little too close to student nihilism. But it is saved by the writing; and The Shield and Dexter are always saved from mere melodrama by their unerring dramatic instincts. The cities are in ruins, the devil is out of his hole, but artistry, for a few hours, makes sense of it all.

The Wire’s fifth and final season starts on July 21 and the second season of Dexter starts tonight, both on FX

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