The classic Cold War film still provides food for thought in the context of a de-personalised and atomised modern society.

Small town America in the mid-fifties. The local doctor, Miles Bennell, played by Kevin McCarthy in the film, is visited by a string of patients who assert that friends and close relatives are impostors, despite the fact that they retain the same appearance, mannerisms and memories. Against all the evidence they insist that the person is not what he or she appears to be.

As the tale unfolds it is revealed that spores floating through space have settled outside the small Californian town of Santa Mira and grown into pods which duplicate and then replace the local residents. Perfect replicas in every respect except that they lack the ambition, drive, determination and overall humanity of those they have substituted for. At one point the former psychiatrist pod person, Doctor Kaufman, explains to Miles, "Think of the marvellous thing that has happened. Seeds drifting through space for years, ... took root by chance in a farmer's field, to offer us an untroubled world... There's no need for love." When Miles responds that this means no emotions, no feelings, only the instinct to survive, Kaufman responds, "You say it as if it were terrible. Believe me, it isn't. You've been in love before. It didn't last. It never does. Love, desire, ambition, faith - without them life's so simple believe me."

The replacement takes place as people sleep and Kevin McCarthy suggested as the title for the film Sleep No More. In the film, though not in the book, Becky, Miles' girlfriend, slips into unconsciousness and has, in a sense, her soul stolen while she sleeps. The climatic scene is when a panic stricken and hysterical Miles runs amongst the traffic on the freeway shouting, "They're here already! You're next! You're next! You're next!".

The story works on several levels. The belief that the world is against you and that friends and relatives are not who they pretend to be would, of course, be evidence of paranoid delusions. It also speaks to an uncertainty about what defines us as human beings. This is also a theme in the Philip K. Dick book Do Androids Dream of Electric Sheep, which provided the inspiration for the film Blade Runner. The unease resulting from the Dolly the sheep affair has the same roots. Would a clone be a human or just a replica of a human?

The film was widely seen as an anti-communist parable. The pod people, with their lack of emotion and determination to succeed in their plan to take over the town were a metaphor for the communist conspiracy. The producer, Walter Wanger, did seem to take this view but Siegel saw the movie as a satire on the conformism of Eisenhower's America and Finney denied any political intent behind the story. Clearly the world of the pod people would be totalitarian in some sense and the attraction of the film was, in part, that it meshed with the Cold War fears and preoccupations of the time.

The film was remade in 1978 by Philip Kaufman. This time the action was transferred, along the coast, from the claustrophobia of small town America to San Francisco with the anonymity, anomie and alienation of big city life. This is a post-Kennedy, post-Watergate conception of the pod people. The leading characters take conspiracies for granted as part of the everyday fabric of modern American life. The fact that one of these conspiracies is extraterrestrial in origin they can easily take in their stride. The first film relied on the fact that people knew their friends and neighbours so well they could suspect that they were not who they claimed to be. In Kaufman's film the impersonal and fractured nature of city life means that people do not really know their neighbours or even their partners, so they might just as well be aliens.

Kaufman has the pod people speak the language of psychology and personal development. They are certainly not communists but more like a West Coast religious cult such as Jim Jones' Peoples' Temple, who committed mass suicide in the jungle of Guyana (having fled San Francisco) in the same year as the film was released. In 1997 Marshall Applewhite's Heaven's Gate followed the same path down the coast in San Diego. This religious cultist aspect of the film is highlighted in a scene where psychiatrist turned pod person Doctor Kibner (Leonard Nimoy) offers lead characters Matthew Bennell (Donald Sutherland) and Elizabeth Driscoll (Brooke Adams) the prospect that they will be, "born again into an untroubled world". More or less what the followers of Jones and Applewhite expected.

In 1994 Abel Ferrara turned Finney's story into a film for the third time, this time with the title Body Snatchers. The location was shifted again to a southern military camp. Shots show US marines quick marching as they chant in monotonal unison. The message is pretty clear. If these people were taken over by aliens how would anyone know?

This is very much a product of the loss of faith in government which has overtaken American society since 1956. The last words in Siegel's original are a psychiatrist ringing the FBI to alert them to the alien menace; in Ferrara's version the military, the bureaucrats and the government are the aliens. When Siegel made his film public opinion polls showed that sixty per cent of Americans considered that the federal government did more good than harm and forty per cent thought the government did more harm than good. By the time Ferrara made his film the proportions were almost exactly reversed.

Negative attitudes towards government, particularly the federal government, have grown apace, particularly since the Watergate Affair of 1973/74. Ronald Reagan was able to tell the American people that the ten most feared words in the English language were, "Hello, I'm from the government and I'm here to help" and this was whilst he was chief executive of that same government.

In 1964, when arch-conservative Barry Goldwater, was running for the White House, American historian Richard Hofstadter identified what he called the "paranoid style in American politics". A tradition which he saw stretching backwards from the anti-communist hysteria of the 1950s and 1960s to similar movements in the 19th century. Politicians convinced that Illuminati, Free Masons, gold bankers or Catholics were seeking to overthrow the United States. Such politicians (mainly of the extreme right) see conspiracies everywhere. Nothing happens by accident or as a result of long term and largely impersonal processes, the hand of the conspirators is seen in all things.

The John Birch Society, for example, argued that President Eisenhower was a conscious tool of the communist conspiracy. The fact that there was no evidence for this showed just how devious, deeply laid and dangerous the conspiracy was.

When Hofstadter was writing conspiracy theories were niche market providers but by the 1990s they had become a mass market. The success of a television series such as the X-Files is indicative of the zeitgeist of modern America. The real enemy is not the aliens but the government engaged in a conspiracy against its own people. A cursory visit to the Internet shows conspiracy theories to be everywhere. Many Americans fantasise that their problems (individual or collective) are the product of malicious plots against them by invisible powers. Gavin Esler has recently reported an anti-abortionist saying to him, in a church in Minneapolis, "the pro-aborts have dead eyes. Look in their eyes. They are dead. They have no souls." Political opponents are seen as not truly human, in this case, literally, the agents of Satan.

For hate groups like Aryan Nations, Christian Identity or the various militias the idea that the government is an alien entity controlled by Israel and the United Nations is a fact taken for granted. Even some more mainstream Republican politicians are attracted to ideas of government inspired conspiracies. For example Congresswoman Helen Chenoweth of Idaho accepts the reports of unmarked black helicopters spying on citizens. She calls on the federal government to stop the flights instead of seeing the black helicopters as something between a fantasy and a hallucination. She probably had lunch with Elvis last week.

Louis Farrakhan's Nation of Islam grabs the headlines with its extreme rhetoric against whites and Jews but few people understand the elaborate conspiracy theory which underlies the message. White people are the product of a seven thousand year old genetic experiment initiated by the evil scientist Yacub. There are even space ships which minister Farrakhan has, of course, travelled on. The Nation of Islam exactly fits Hofstadter's concept of the "paranoid style". The movement, for all its would-be outsider status, is as American as apple pie.

A belief in malevolent conspiracies may make harmless or even entertaining television of the X-Files or Dark Skies variety but combined with politics it is a dangerous and combustible formula. Timothy McVeigh bought into conspiracy theory when he bombed the Alfred P. Murrah Federal Building in Oklahoma City in April 1995, killing 168 people, including 19 children. A former friend of McVeigh's put it like this, "He considered all those people to be as if they were the stormtroopers in the movie Star Wars. They may be individually innocent. But because they are part of the evil empire, they were guilty by association." It is as if McVeigh saw his victims as pod people, not really people at all and a threat to his, authentic, humanity.

Finney's novel is a powerful tale and it has inspired one brilliant film (Siegel's) and two very good ones. It is story which can be reinvented and reworked as times change and so too do our preoccupations and concerns. The pod people are just like us, exactly like us, but the meaning has been removed from their lives. They offer the same jobs, the same cars, the same houses but a materialism emptied of any spiritual or emotional content. They offer, in short, the kind of normless, homogenised, secularised and globalised society which threatens to be our fate.

As Kevin McCarthy put it "They're here already!" Perhaps the real problem is that the depersonalised pod people are already here. They're us.

All three versions of the film are available on video. Astonishingly Finney's book is out of print in Britain but can be ordered over the Internet from Amazon.