In theory, Garbiele Muccino’s family-friendly drama “The Pursuit of Happyness” should be a puke fest of sticky-sweet sentiment and positive values.

In practice, however, it’s a moving tribute to triumph in the face of adversity that toes the line between hard drama and Hollywood fluff with a deftness Disney can rarely muster.

“Happyness” finds Will Smith in pursuit of the American dream, not to mention an Oscar, as Chris Gardner, an enterprising but hapless father struggling to eke out a living as a small-time salesman in San Francisco. When opportunity knocks in the form of an unpaid, six-month internship program at Dean Witter, Gardner seizes his chance even as his wife splits for New York. Soon he’s homeless, struggling to be the candidate chosen for a stockbroker job while simultaneously raising his toddler son.

The film is “inspired by a true story,” which in most cases would mean diced up, reassembled and glazed over with enough schmaltz to kill any semblance of truth.

In the realm of big-studio blockbusters, true stories are generally about as “true” as “The Real World” is real.

“Happyness” proves to be the exception to this rule, maintaining a realistic grittiness in the telling of its one-man-against-the-world fable. Is it romanticized? Sure. Has it been tummy-tucked, touched up and made over for mass consumption? Undoubtedly. But what’s left is mostly the good stuff.

That’s not to say the film follows any sort of radically new approach. The story is conventional rags-to-riches all the way: Man doesn’t get any respect as impoverished failure. Man takes a chance on the long shot. Man devotes life to goal, as is summed up in heartwarming montages.

We’ve heard this story before and learned its lesson well. As Smith himself says in one of the blessedly few inspirational speeches in the film, “You got a dream, you gotta protect it.”

But rarely have we seen it played out with such intensity. “Happyness” may have a cutesy title (the misspelling comes from a mural outside a Chinese daycare in the film), but the content is by no means sugarcoated.

Muccino paints a relatively straightforward portrait of poverty, and even touches on the ugly issues of domestic strife, less-than-ideal child rearing and bigotry in the workplace. The maker of “The Last Kiss” directs with a firmly grounded panache and self-restraint that keeps the movie from toppling into the abyss of “Jungle 2 Jungle”-style family fare.

Nearly all the unlikely success achieved by “Happyness,” however, can be attributed to its acting.

Thandie Newton gives a spot-on performance as Gardner’s wife, Linda, carefully imbuing the easily hated character with enough candor to engage our empathy, even as we side with Smith.

And Gardner’s son is played with believable earnestness by Smith’s real-life son Jaden. The real-life connection only helps the father-son bond that essentially drives the film, as the Smiths have an unforced chemistry on screen. No nepotism here, however: Jaden earned his spot through auditions. Jaden seems to have inherited some of Smith’s acting genes, playing the kid with a passable combination of naiveté, child-like wonder and innocence lost. Sometimes he’s downright bratty, which only adds to the movie’s realism, since what kid wouldn’t be grumpy living out of a homeless shelter or train station?

But the film’s real tour de force is the older Smith, who proves he belongs in that small cadre of performers who have truly graduated from comedy and action schtick to genuine drama acting. “Happyness” is Smith’s show from start to finish. For the film, Smith adopts the role of Gardner effortlessly, hiding his boyishness behind a grizzled exterior.

Right from the beginning, we see the charm that will be Gardner’s key to success, existing uneasily alongside a profound frustration with the dehumanizing poverty he fights against daily.

As bills mount and the pressure builds, Smith becomes gradually more irritable, letting us see the effects on Gardner’s resilient spirit as he recognizes the corner in which he is trapped. The transformation manifests itself in outbursts of yelling at his son, as well as a subtle withdrawal into his inner self. By the end of the film, Gardner can only smile as his superiors joke with him, completely oblivious to his struggle. Smith throws in just enough cynicism and anger to sell the movie while not veering too far off the family film path.

For the most part, the script aids him in this, although Smith’s narration is a little too cute for its own good. At each turning point, Smith proclaims, “This part of my life is called [blank],” replete with pauses for comedic effect.

Other gimmicks include the frequent loss and theft of the bone density scanners Gardner is trying to sell. But while most running gags on the family comedy/drama side of things would make you cringe, in “Happyness” they’re often just funny enough to induce chuckles and just poignant enough to endure throughout. Quirky characters further add to the movie’s charm, which would be sickeningly sweet without such a twist.

All of these combine to make the film enjoyable, if not exactly hard-hitting: Muccino never fully addresses the questions he raises about the culture Gardner is buying into. The corporate brass is portrayed as ignorant but benign. Racism is hinted at when Gardner’s superior repeatedly uses him as a gofer, but remains a faint whisper of a theme.

On the whole, however, the film gives a warts-and-all picture of crushing poverty with more meat than most comparable films.

It’s a marketable formula — “Happyness” hit No. 1 at the box office — that still manages to engage viewers and even raise a few discomforting questions about the underbelly of America and its elusive dream. More films should be like “Happyness,” not just in theory but in practice as well.

Grade: 3.5 out of 5