It’s loosely based on a true story, one that suggests a splendid way to pay off student loans: “21” follows some MIT brainiacs as they learn to count cards and flourish at Las Vegas blackjack tables.

Their leader is a cynical teacher, played by Kevin Spacey, who clearly doesn’t need to worry about tenure. Not when he makes thousands of bucks with every trip to Sin City.

Our hero is Ben (Jim Sturgess, the newly-minted heartthrob from “Across the Universe”), a reserved lad but a math genius. We haven’t seen such a movie whiz-kid since Matt Damon in “Good Will Hunting.”

Ben is quickly drafted into the elite undergrad circle cultivated by Spacey’s prof. Conveniently, this group includes the dream girl (Kate Bosworth) Ben has been daydreaming about.

You can pretty much program out the rise and fall of a plot like this from there: success, danger, life lessons. Is it possible the glitz and glamour of Las Vegas will go to the head of our naive hero?

Things get especially sticky when an old-school casino cop (Laurence Fishburne) takes an interest in Ben’s mysterious success at cards. Card counting isn’t illegal, but the movie tells us that casinos have a way of “discouraging” the practice, using a few handy Guantanamo Bay methods.

We keep hearing that Fishburne’s style of cop is being replaced by automated surveillance systems, so you can assume that he will prove his worth before the movie’s over.

The plot sounds silly, but “21” is silly on top of that. Director Robert Luketic (“Legally Blonde”) can’t seem to take the story seriously, and maybe he shouldn’t. Every time somebody tries to get serious, the whole cast looks like they’re ready to break out in giggles. How Kevin Spacey keeps a straight face is a mystery — these days his movie career seems to consist of taking random juicy roles during breaks in his stage career.

Needless to say, “21” should inspire a generation of young people to count cards at blackjack. Who says you can’t learn from the movies?