“The Pursuit of Happyness” is the biography of a real guy named Christopher Gardner, whom Will Smith, charming and bright, embodies to the fingertips. As the film has it, Chris is (a) extremely likable, (b) a whiz with numbers and (c) a total loser.

And why is Chris a loser? Well, wanting his piece of the dream and trusting in his salesmanship, he has invested in bone scanners about the size of a sewing machine and trundles about the San Francisco area (it’s the early 1980s) trying to sell them to physicians. The lack of success is grinding him down, down, down; he’s not moving a single unit. The rent is due. He’s behind in all his other payments. His wife (Thandie Newton) has left him, he has been kicked out of his apartment and he’s on the streets – with his son.

That’s what makes it all the more painful to watch: The proud man who had dreamed for so much is humiliated not merely by his failure but by the fear and pain it inflicts on his son (played beautifully by Smith’s own son Jaden).

So “The Pursuit of Happyness” is basically an account of Chris’ ordeal by internship. On a whim, he walks into the San Francisco Dean Witter office and learns that every six months, the fancy brokerage takes on 20-odd interns for a six-month training period, salary $0.00. At the end of it, the guys in the suits may or may not hire one survivor. Chris decides he will be that one.

Will Smith, right, and his son Jaden star in the biography The

The movie is almost devised like a rat-in-maze experiment at the Yale psychology department. Each few minutes some new obstacle comes up for Chris, threatening to obliterate his dreams, at which point the film stands back and watches him improvise brilliantly on the run.

What keeps him going? Seems to be faith in self, always underestimated, yet one of the more telling strokes is the movie’s – and Smith’s – refusal to idealize Chris too far.