And perhaps, back then, lines that simply substituted 'ape' for 'man' and 'simian' for 'human' in familiar epithets were considered clever. Now, clunkers such as 'the proper study of ape-kind is ape' (Alexander Pope), 'he never met an ape he didn't like' (Will Rogers) and a reference to 'the milk of simian kindness' weigh down an already cumbersome script.

Based on the 1963 novel by Pierre Boulle, the plot is driven by Taylor (Heston), an astronaut leading a voyage into deep space. Painstakingly, it is explained that although the crew have spent a mere six months out of orbit, a bending of the space-time continuum means 700 years have passed on earth.

The year is 2673 when we meet Taylor, wondering aloud from the captain's seat of the spacecraft, 'Does man, that glorious paradox who sent me to the stars, still make war against his brother and keep his neighbour's children starving?' Stirring stuff, no doubt, for a society just catching their breath after the peak of the cold war.

Taylor and crew crash-land into a lake in a landscape that looks quite a lot like New Mexico. As they watch their rocket sink into the water (breathing perfectly well in this new atmosphere), the captain announces 'OK, we're here to stay' and lights a cheroot (as you do).

The men discover the planet is ruled by apes who ride horses and enslave a race of mute humans for scientific study. They are captured. Taylor's companions go missing for the rest of the film and he is left to negotiate his way out of the clutches of the evil Dr Zaius, an orangutan (Maurice Evans).

Orangutans, we learn, stand for white people and chimpanzees (who are accepted to Zaius' research institute by a quota system) for black people. This is irrelevant except that it provides one more layer of social commentary to a film that is already rather overdoing it.

Anyway, a scientist-chimp named Zira (Kim Hunter) and her fianc?, Cornelius (Roddy McDowall), learn that Taylor can talk when he utters his seminal line during a round-up of the captive humans: 'Take your stinking paws off me you damn dirty ape.' The chimps set about proving their theory of evolution (in this case that apes evolved from the lower-order primate man), which is at odds with the orangutans' creationist teachings.

Zira, Cornelius and Taylor are, after great tedium, exiled to the planet's Forbidden Zone, where they uncover relics of a lost human civilisation. Now, the ending starts to creep up on you. It's a brilliant twist executed beautifully, despite the obvious limits of the era's cinematographers.

Taylor has gained his freedom and, with his woman Nova (Linda Harrison, above right with Heston, who has no purpose other than to pout and look fabulous in animal hide), is riding across a Forbidden Zone beach when the astronaut realises where he is - where he's been all along.

He stops the horse. A look of horror crumples his face. The camera pans down across a fan of grey-green spikes and the steel outline of a flaming torch.

Taylor leaps off the animal and kneels in the water. 'You maniacs!' he cries, 'You finally did it. You blew it up! God damn you all to hell.'

The parting shot of the Statue of Liberty lording over a man weeping into the sand remains, despite - or because - of the film's age, as powerful and frightening as ever.