The last of this festival’s trio of the late, great Hal Ashby’s 1970s films brings us gifts of the equally late and talented Peter Sellers in “Being There,” taken from Jerzy Kosinski’s novel. It’s a sweet, totally unbelievable, happy-face fable from long ago and far away.

“Being There” gives us Sellers as Chance, a middle-aged, mentally deficient man who has been living his entire life in a well-kept mansion set in a decaying neighborhood that, unbeknownst to Chance, is virtually a moonscape.

Chance knows nothing of that? He didn’t hear the drills and bulldozers just outside his door? It’s a fable.

Chance’s only access to the outside world has been by watching television, with a menu ranging from Johnny Carson to Mr. Rogers.

Bit by bit, drip by drip, we learn that Chance has never been in an automobile or elevator, grocery store or airplane. He has never used a telephone or talked to another human being other than Louise, the maid in the house. He can’t write or read.

The owner of his house, a wealthy recluse who only appears as a corpse, apparently has kept Chance as a “guest” since childhood. Was Chance the product of a romance gone bad? That would have made an interesting movie.

Chance, who has for about 50 years been wearing the old man’s fine clothing from the attic, has acted as a gardener, tending a tiny patch in the back yard. So “Gardener” becomes his last name.

Ashby skips over the details of who notified the authorities and buried the old man. As there was no phone in the house, did Louise walk to a pay phone? OK, it’s a fable.

One fine day, two visiting lawyers arrive to tell Chance he has to leave so the house can be torn down. Chance steps out into the world with his ancient, expensive but dated clothing, a gorgeous alligator suitcase, a fancy overcoat and an umbrella — but no cash.

Chance, for the first time in his life, gets to see his neighborhood. It’s Warsaw after the Nazi garage sale. We’re shocked. Chance is bemused, but moves on.

By nightfall, tired and hungry, Chance steps off a curb and is banged up by a chauffeured limo. The passenger is Eve Rand (a very young and delicious Shirley MacLaine). The injury to his leg is minor, but Ms. Rand insists on taking him home, so that her in-house doctor (Richard Dysart, who grew up in Skowhegan and Augusta) can examine him. Remember, these are the ’70s as imagined by Hollywood.

“Home” is a mansion high in the wooded hills that has everything for basic survival: 40 rooms, each with a television; six kitchens; two elevators; dozens of servants and a state-of-the-art hospital area, where Eve’s ailing trillionaire husband, Benjamin Rand (Melvyn Douglas, but think David and Charles Koch), can be taken care of.

Chance’s childlike innocence charms everyone he meets, including Benjamin, who invites him to move in.

Father Rand introduces Chance to his fellow all-white, old-guy Republicans, including the president of the United States (Jack Warden), who is so numb he offers Chance a cabinet position. Chance’s fame grows so rapidly among Republicans, he is considered presidential material.

This goes to show you that nothing in politics ever changes.

One shouldn’t be surprised to find that Sellers tried unsuccessfully for years to make this project happen. But when his “Pink Panther” series brought him big money, he got the studio to front the film.

All went well. The movie did well, Sellers won the Golden Globe, Ashby took home the Golden Palm from Cannes, and Melvyn Douglas won Best Supporting Actor.

“Being There” is full of both holes and great comic scenes, most of which wouldn’t pass the wince test today. But then again, it’s a fable.

The best would be MacLaine’s attempt to seduce Chance while he is watching a “Mister Rogers’ Neighborhood” episode. While she smothers him with sexual moves, the channel turns to Steve McQueen making love to Faye Dunaway in 1968’s “The Thomas Crown Affair.” Chance’s eyes never stray from McQueen, and he apes his moves. It doesn’t work. “What’s wrong?” a tearful Eve asks.

MacLaine’s Eve, who “favors older men,” is wonderful, sexy, svelte and rich, but we suspect just a tad “slower” than Chance. One could do an entire column on her character.

Melvyn Douglas as her dying husband is heartbreaking in this, his last role, for which he was awarded the Oscar.

Many think that this was Sellers’ greatest role. While he does a superb job of making a monosyllabic stranger charming, Chance is clearly not Sellers’ best work. Most would agree that that honor would go to his three roles in Stanley Kubrick’s “Dr. Strangelove.”

I’ve seen some great Hal Ashby films, including “Coming Home” and “The Last Detail.” This hasn’t been one of them.

J.P. Devine, of Waterville, is a former stage and film actor.