‘A strange neurosis… evidently contagious… people worrying about what's going on in the world’, that is how the contented, pipe-smoking, 1950s Californian small town psychiatrist explains what is happening in Santa Mira. Ten or more patients referred complaining of their relatives somehow being changed – and the hero, a local doctor/GP played by the strong-faced actor Kevin McCarthy (a great name for a McCarthy era paranoid thriller) cannot work out what is going on. He is first seen at the city emergency hospital, as a screaming, dishevelled paranoid psychotic (‘you fools I'm not crazy… I'm a doctor too… you must understand me… I'm not crazy’). He recounts the story in a prolonged voice-over flashback. The audience is a sceptical emergency clinic physician, a policeman, and ‘Dr Hill from the state mental hospital’. Is Kevin mad, even though he starts to talk quite calmly, or are there really large leafy pods turning into people and replacing them with emotionless automatons, looking like people but with ‘something missing’?

Given the liberal way our doctor hero hands out tablets, some sleeping meds to young Jimmy who has ‘got the crazy idea his mother isn't his mother’, as well as things to keep awake on, for him and his beautifully coutured girlfriend (her strapless dress is a joy to behold) you might well think he is part of the problem. Were the film to be part of a psychiatric exam you would mark down anyone who did not put amphetamine-induced psychosis at the top of the differential diagnosis. However, as he and the story unravel we delight in the clothes, cars and cocktails of middle-class America, and the nightmare dislocation of what happens when ‘something evil has taken possession of the town’, with people having ‘no difference you can actually see’. Increasingly perplexed, the doctor hero faces cancelled appointments, a friend having ‘I guess you'd call it a delusion’, such as to think ‘Uncle Ira isn't her uncle, he's an impostor, someone who only looks like Uncle Ira’, increasing alarm as to who to trust, and of course casual references to his friend the local psychiatrist as a ‘witchdoctor’. Eventually he has to flee somewhere into the surrounding hills, clinging onto his girlfriend who bravely runs along behind him in high heels. Before this he admits, ‘I'm just a general practitioner, love is handled by specialists’.

Fast-paced, economical, black and white, a mere 80 minutes long, Invasion is not just a classic paranoid thriller, it is also an accidental archive of the 1950s medical practice and a brilliant exposition of the Capgras syndrome, en masse. Most viewers read it as either a trumpet call to the dangers of a communist takeover (‘there's no emotion, just the pretence of it’) or a coded warning of the dangers of McCarthyite witch-hunts (‘is he Uncle Ira or isn't he Uncle Ira?’). Not overtly a film about psychiatry or its practices, like so many police and sci-fi melodramas it uses a psychiatric condition as the framework for plot, character and exposition. As writers became more confident with this kind of material, drawn from our very own casebooks, they no longer needed a doctor or psychiatrist actually in the picture. The investigating cop, for example Jack Nicholson in Roman Polanski's Chinatown (1974), unravels the history and diagnosis thanks to his intuition and police know-how, but he was just dealing with child abuse.

However you take Invasion, it is not only fine entertainment, but also great fun for psychiatrists to watch. Its implicit assurance that psychiatrists probably know much more than anyone else about how the world works and why people do certain things is also rather refreshing. After all, ‘Dr Hill from the state mental hospital’ does let our hero have his say.