21 — ** — Jim Sturgess, Kevin Spacey, Kate Bosworth; rated PG-13 (violence, profanity, brief sex, vulgarity, slurs, brief partial nudity, brief gore)

Like a seriously troubled gambling addict, "21" shows its hand too early and then stays at the table too long for its own good.

This fact-based piece runs two full hours, and by the time it's done, some huge holes in its plotting have been exposed and a ridiculous, laughable ending has been tacked on.

That's a shame, because the first hour of the movie is actually pretty good. It's based on "Bringing Down the House," the nonfiction best-seller about how a half-dozen Massachusetts Institute of Technology students used card-counting techniques and player-communication strategies to take millions from Las Vegas casinos.

The fictionalized version stars Jim Sturgess ("Across the Universe"). He plays Ben Campbell, a brilliant and hard-working but cash-strapped student who's been accepted to Harvard Medical School once he completes his undergraduate work.

Unfortunately, he doesn't have enough money to pay the Harvard tuition. And his chances of getting a scholarship are remote at best.

So, he agrees to join a "team" led by an MIT professor, Mickey Rosa (Kevin Spacey). He's gathered some of the school's best and brightest, and has been taking them to Sin City on weekends to gamble at the blackjack tables.

Director Robert Luketic ("Legally Blonde") and two credited screenwriters do a good job of explaining the fictional Rosa's gambling "system" and techniques. The plotting is a little muddied and is pretty sketchy towards the end, though.

And Sturgess does try to cover up his natural British accent with an unconvincing New England one. But he's a pretty likable hero, even when the film demands otherwise.

As for Spacey, he chews so much scenery here that you expect to see plasterboard between his teeth. And Bosworth is a pretty negligible presence.

"21" is rated PG-13 for some strong violence (including a brutal interrogation scene, some fisticuffs and brief gunplay), occasional profanity, a brief sex scene, some suggestive language (slang references), derogatory slurs, brief partial female nudity, and brief gore. Running time: 122 minutes.

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E-mail: jeff@desnews.com