These days, a lot of young actors are too attractive, or at least too confidently invested in the powers of their teenybop charisma, to do a convincing job of playing anyone ''normal.'' They may look great in tabloid party photographs, but when they're required to suffer or to show anxiety — that is, when they have to behave like the rest of us — they're like automatons who've been to acting class. They turn imperfection into dull posing.

Jim Sturgess, the star of the college-brainiacs-go-to-Vegas drama 21, is in a different league. It's not that he isn't cute. He looks a little like the young Paul McCartney (that must be part of why they cast him in Across the Universe), with a boy heartthrob's dewy eyes and friendly, puckered grin. In 21, though, he attaches real live nerve endings to that pretty-boy facade. He plays Ben Campbell, a math nerd at MIT who's recruited, by a wry Mephistopheles of a faculty member (Kevin Spacey), to join a secret team of student blackjack wizards who head to Las Vegas on weekends to rake in hundreds of thousands of dollars by counting cards. Sturgess wears his hair in a longish, haphazard cut that's like a floppy helmet. It's the armor of a kid who's shy about everything but his intelligence. As Ben, he's passive and slightly dorky, a gummy collegiate tangle of sweetness, IQ, and loser psychology. Ben hangs out with a couple of geeks (together they're building a robot), and he takes it as a fact that girls, or at least the hot ones, aren't interested in him. But when he sits down at the blackjack tables, that very hesitancy — his reluctance to reveal himself — works for him. His brainy reserve becomes cool, a way of negotiating risk. He's a Will Hunting who turns into James Bond.

I've never quite gotten how counting cards in blackjack works, but 21, which is loosely based on the book Bringing Down the House, about the experiences of MIT student Jeff Ma and his junior-gambler colleagues, constructs an entertaining, if simplified, version of the strategy, to the point that some viewers may be tempted to use the movie as a primer. Under the tutelage of math-and-stats professor Micky Rosa, played by Spacey with his familiar — but always bracing — joy-buzzer sarcasm, Ben joins the other students in an elaborate system that involves assigning a number to each table based on how many low (or high) cards have been dealt. One team member, usually a girl, is there to signal when a table ''heats up'' (i.e., when most of the high cards have yet to be played). At that point, a designated Big Player can sit down and start winning.

21 is built around some standard-issue plot mechanics, but it's still a clever and novel card-sharp thriller. It draws us into Ben's excitement at a luxe dreamworld that trumps college. For Ben, the blackjack team is the means to an end, the only way he has of making the $350,000 he'll need to attend Harvard Medical School. What the audience sees is that the danger of walking into a Vegas casino to beat the system gets him more jazzed than he can admit. High on his big winnings, he starts to get high on everything else too, like the attention of a sexy teammate — Kate Bosworth as an ice princess at full melt. (Bosworth makes her lusciousness vivid in an underwritten role.) He's high on sin. Counting cards is, in fact, perfectly legal. If the casinos catch you, however, they'll kick you out — or, as the film keeps reminding us, break your nose. And since the ability to count cards is truly a skill, not just a way of cheating (it is, of course, the fabled essence of what makes a great poker player), Ben is lying — and not lying. He occupies a gray area of the sleaze zone that, in the movie's terms, is a metaphor for his ambivalence about being a player in life. He desires it and fears it.

21 is a better realized youth thriller than, say, Disturbia, but it has more things to niggle at than it should. When Ben gets to Vegas, he and the other students are given disguises and fake identities, but the movie barely follows through on this gambit — a missed opportunity. And there's a key turning point when Spacey's prof, who seems mostly amused at the scam that he's orchestrating, blows up at Ben in a way I never bought. Despite its shortcomings, however, 21 has enough good twists to keep jolting ahead. It's too early to tell if Jim Sturgess is going to turn into Ryan Gosling or Andrew McCarthy, but my bet is that he gets closer to the former. The fun of 21 is the way that this sharp, hyperaware star in the making, his face as readable as a mood ring, pours us into an adrenalized cocktail of fear, desire, and mental buzz.