Among the generation of directors who tried to turn Tinseltown on its head during the ’70s, Hal Ashby holds a special place. In Peter Biskind’s account of the era, Easy Riders Raging Bulls, Ashby becomes almost a metaphor for the whole New Hollywood adventure: barrelling in from left-field in the early-70s with a string of odd, small, unexpected movies brimming with a new kind of scattershot energy; run aground by the end of the decade, washed up on the tidal wave of drugs and studio frustrations, burnt out and out of ideas.

Ashby directed eleven movies before his untimely death, from liver cancer, in 1988. Mainstream acclaim came with the sixth, 1978’s post-Vietnam melodrama, Coming Home, picking up Oscar nominations for best picture and best director. In truth, though, that’s a hard film to watch today – stiff and self-conscious in its hand-wringing, button-pushing and message-delivering, and kept alive really only by the slashing, jagged, resent provided by Bruce Dern.

Ashby’s shaggy cult really rests on four films: Harold and Maud, The Last Detail, Shampoo and – the last weird flash before terminal decline – Being There, released, significantly enough, in the first month of 1980. Revisit that quartet, though, and you’ll run into the central conundrum of Ashby’s career: it’s hard to locate the director’s presence or personality at all. Instead, the success of all four films depends on the writing and, in particular, the lead actor – none more than Being There, which frames Peter Sellers at his most restrained yet most brilliant, soloing with intense, intensely quiet genius in his penultimate role.

Adapted from the book by Polish-born novelist Jerzy Kosinski, it’s a modern-day fable, a spin on the old tale of the emperor with no clothes, about a simpleton who gets mistaken for a political guru. Sellers plays the void at the centre, a middle-aged man called Chance. His origins are mysterious: all we know is he has lived his entire life inside the townhouse of a wealthy old man in Washington DC, and that there is something not quite right about him. As Lousie, the elderly black maid who has kept him clothed and fed all his life puts it: “No brains at all. Stuffed with rice pudding between the ears. Short-changed by the Lord, and dumb as a jackass.”

For decades, the child-like Chance has tended the old man’s garden, but he has never ventured beyond its walls. His only contact with the world beyond comes via the TV set whose channels he obsessively, eternally surfs, staring uncomprehending, sometimes mimicking actions, but taking little in before flicking on to the next station.

One day, though, the old man dies, Louise leaves, the house is sold and Chance – dressed like a gentleman in the clothes he has inherited from the dead man: natty hand-tailored 1930s suit, overcoat and homburg – finds himself exiled from the garden, cast out into the fallen world outside, armed with only his TV’s remote control. Sellers does a lovely job here, stumbling politely from his quiet mahogany sanctuary, out into the loud, derelict, dirty and profane streets, while Eumir Deodato’s funked-up working of “Also Sprach Zarathustra” blasts on the soundtrack, unavoidably stirring associations of Stanley Kubrick and Sellers’s work with that great director.

With Sellers clicking his remote to try and get rid of a gang of youths who confront him, Ashby has great fun with this sequence, but the big joke is still to come. Following a chance encounter, Chance the gardener – mistaken for a businessman and rechristened “Chauncey Gardiner” – is swept into the elite, cloistered world of Ben Rand (the veteran Melvyn Douglas, who won an Oscar for his work here) a dying industrialist billionaire and political kingmaker on the Washington scene, known as the President’s closest confidante.

Sitting by Rand’s side, Chance’s silence gets mistaken for wisdom. When he pressed for comment, he talks meaninglessly about the only thing he knows about: “As long as the roots are not severed, all is well. And all will be well in the garden.”

It is, as Lousie, watching Chance’s rise from a distance, comments, gobbledegook. Blinded by their own sophistry and the fear of being seen not to understand, however, the political elite, and then the audiences watching on TV, seize on his gnomic utterances as pearls of profound, folksy wisdom. Before long, as people constantly read their own meanings into Chance’s uncomprehending silence, the gentle idiot is being feted as a new sage, and carried to the verge of assuming the presidency of the USA.

This was a deeply personal project for Sellers. He lobbied long and hard to get the part; indeed, when the novel was first published in 1971, Kosinski received a telegram bearing the message “Available in my garden or outside of it,” signed “Chance the Gardner.” When he dialled the phone number accompanying the note, Sellers answered. By the time he finally got to play the part, the actor was ill. Being There would be the last film of his to be released before his death, and, although nominated, he was reportedly devastated when he failed to win the Best Actor Oscar in 1980.

The real reward, though, lies in the incredible performance he left. Sellers has an impossible job here: to play a cipher, a man who isn’t there, and yet imbue that void with fleeting hints of something profound. Somehow, he pulls it off. With his little hat and umbrella, he is as absurd, unsettling and enigmatic as a figure from Magritte. The performance looks more incredible as the years pass. Stupid, still and silent, he simply radiates.

This is Peter Sellers’s film, as much as The Last Detail was Jack Nicholson’s. It’s simply impossible to imagine the movie with anyone else. Still, it was Ashby who came up with the last, magical touch that pushes the thing towards being a masterpiece : the enigmatic final scene when, unobserved by anyone else, Chance obliviously begins to walk on water, strolling curiously across the surface of an autumnal lake.

With its constant, overloading backdrop of flickering screens, you could say Being There was prophetic in its view of the way the media would go in the USA, Chance as the figurehead of a generation who gave up reading and thinking. You could even say that, in recent political life, we have seen more cynical, terrifying, versions of the plot being repeated, worse each time.

Watching that final scene, though, it’s impossible to work out what it is supposed to convey. The moment sends shivers running back through the movie, retrospectively altering what you saw, connecting with the Biblical references Kosinski litters through the plot – but the scene doesn’t exist in Kosinski’s book. (To deepen the irony, Kosinski was later accused of having plagiarised his entire plot from an earlier Polish novel). Ashby apparently improvised that final image on set, inspired by a moment when he saw Sellers’ reflection in a polished wooden floor.

Pure chance, or, maybe, pure Chance. Hal Ashby is not around for comment, but he probably couldn’t tell you what it’s supposed to “mean” either. Still, it feels like it should mean something, and, if you like, you can probably read your own deep significance into the silence of it. Kind of perfect, really.