The central conceit of Griffin Dunne’s “Fierce People,” adapted from Dirk Wittenborn’s novel, is derived from that favorite liberal and multicultural principle — that underneath our superficial differences from even the most exotic of peoples, we are really all just the same. And, in the case of particularly warlike savages, it’s rich people who are just the same.

Do you think you can see where Messrs. Dunne and Wittenborn are going with this?

Finn (Anton Yelchin) is a boy of about 17 who has never met his father, a famed anthropologist working among the primitive Ishkanani — the name means “fierce people” — in the South American jungle. One day, in the summer of 1980, his dad writes to invite Finn to do fieldwork with him in the jungle. Finn is excited about this, but before he can go he is busted in a raid as he attempts to buy drugs for his addicted mother Liz (Diane Lane). He refuses to rat her out. But he suddenly finds himself bailed out by a billionaire named Ogden C. Osborne (Donald Sutherland), allegedly the seventh richest man in the world, whom Liz, a masseuse, has met in the course of her work. Mr. Osborne not only “takes care of” Finn’s drug charge, he also offers both Finn and his mother a place to stay for the summer on his vast estate, Vlyvalle, in New Jersey.

We are never told exactly what business Osborne is in, apart from learning that the old man “makes his pear brandy just the way they do in France.” Somehow I don’t think you get to be the seventh richest man in the world by doing that. But we are told where the Osborne fortune originated, and — just as Balzac said — it turns out to have been in a forgotten crime.

Except that the crime is not forgotten, but remembered proudly by Osborne himself, whose father had employed his mother to seduce the owner of a telegraph company, then stole the company by blackmailing the man with an incriminating photograph of the couple.

That sounds to me like something beyond even the bloodthirsty Ishkanani, but then I’m not an anthropologist.

At Vlyvalle, Finn is initiated into the ways of the sybaritic and conscienceless rich. He forms an attachment to old Mr. Osborne and also to his grandchildren, the comely Maya (Kristen Stewart), a girl of about his own age, and the manly Bryce (Chris Evans), who studied anthropology at Harvard and admires Finn’s father, whom he calls the “Elvis of anthropology.”

Bryce is the kind of preppy guy who goes in for hot-air balloon racing and who pays tribute to Finn — naturally a very cool teenager — by calling him “a gentleman and a scholar.” After a day or two Bryce proclaims “lifelong friendship” for Finn and warm approval of his relationship with Maya. “Welcome to the tribe, brother,” he says.

While Finn learns the ways of the tribe, mom joins Alcoholics Anonymous and begins to get her life together. Old Osborne introduces her to the people of Vlyvalle as his doctor. When she protests, he says: “For 10 square miles, I’m king. If I say you’re a doctor, you’re a doctor.”

All is going swimmingly until something terrible happens to Finn. I have no intention of spoiling the movie for you by giving away the ending, but you would have to be most innocent in the thinking of today’s movie culture not to know instantly who is going to turn out to be the bad guy.

The only question that remains, then, is whether Finn can learn from his studies of the Ishkanani and take a condign revenge on this person and so redeem his nureshi, or tutelary spirit.

Yet here the film lacks the courage of its own left-wing convictions. It is way too glib to say that these highly sophisticated rich people are “really” just like the savage Ishkanani. If you’re going to say that, and if, further, you’re going to show dear little Finn becoming Ishkananized by them, and by what’s happened to him, then you’ve already cut him loose from the enlightened conscience and sense of social responsibility that commercial filmmakers, rightly or wrongly, think Finn has to retain in order to win the sympathy of the audience.

There the movie loses its nerve, allowing Finn in the end to look down upon the Osbornes from a position of moral — and political — superiority, just as Mr. Wittenborn himself does. I don’t know about you, but I’m not buying it.