Will Smith is perpetually underrated as an actor because he rarely challenges himself: For every "Ali," he seems to make five or 10 movies like "Bad Boys II" or "Wild Wild West," which trade on his relaxed affability but don't require much more than a rakish grin. "The Pursuit of Happyness" (and yes, I'll get to the spelling in a minute) therefore comes as a pleasant reminder that Smith, when called upon, can deliver some rather more complicated goods. As Chris Gardner, a struggling single father in '80s San Francisco, Smith looks haggard and weary; you occasionally hear a bit of a tremble in his voice. He's on the verge of breaking down but is determined to hold it together for the sake of his son.

The real-life Chris Gardner, on whose story the film is based, overcame poverty and homelessness to become a successful stockbroker. (His memoir, also called "The Pursuit of Happyness," was recently published. Some significant changes have been made between Gardner's real story and the movie version.) In the film, which begins with Chris' wife, Linda (Thandie Newton), on the verge of leaving him and their 5-year-old son, Christopher (Jaden Christopher Syre Smith, Will Smith's real-life son), we watch Chris — then a not-too-successful salesman of medical instruments — talk his way into a stockbroker internship at Dean Witter, only to find that it is unpaid.

Determined to climb the ladder and make a better life for Christopher, he accepts the internship, and we watch the father/son duo move from apartment to motel room to homeless shelter, with a few nights in a men's room in a BART station along the way. Throughout, Chris is haunted by a mural outside his son's day-care center, which includes the Declaration of Independence's phrase about man's inalienable right to life, liberty and the pursuit of happiness. It's misspelled on the mural as "happyness," which annoys him — he wants his son to learn things correctly — but also seems to serve as a challenge.

Smith creates a palpable warmth with his son; his Chris is motivated by a fierce desire to keep the world safe for his precious child. Jaden Smith (now 8 years old, and a charmer) is a natural onscreen, even throwing a convincing tantrum at one point: This little boy is a nice kid but no angel. Their performances are the reason to watch the movie. We know exactly where "The Pursuit of Happyness" is going, and some of the plot devices created for the movie by screenwriter Steve Conrad (such as Chris constantly managing to lose — and find — the instruments he sells, and being a Rubik's Cube whiz) feel a bit cute.

But director Gabriele Muccino finds the heart of the film in the bond between Chris and Christopher, and in Chris' realistically depicted downhill trajectory; poverty, here, is as close as a missed sale. Ultimately, the real Chris Gardner emerges as an inspirational American success story — and Will Smith emerges as a fine and nuanced actor.

Moira Macdonald: 206-464-2725 or mmacdonald@seattletimes.com