Perhaps the most fascinating aspect of the movie "21" is that it is based on a true story.
Beginning in the '80s (and continuing to operate well into the next decade) there were students from M.I.T. who participated as card-counters and managed to fleece the Atlantic City and Las Vegas casinos for millions. These activities spawned the book "Bringing Down the House" on which this movie is based.
Ben Campbell (Jim Sturgess), a shy, but brilliant undergrad at M.I.T. has a goal to get into Harvard Medical School. His chances of winning a highly contested scholarship are slim and none. One of his teachers is Professor Mickey Rosa (Kevin Spacey) who attempts to recruit him into a highly secret team of card-counters Rosa has organized. At first Campbell declines, but then reluctantly agrees to join the group as a means toward an end -- when he makes $300,000 (the amount he needs for Harvard) he will stop.
One of the team members is beautiful Jill Taylor (Kate Bosworth). Her role in the scheme is to act as sort of a lookout at the blackjack table to alert the counter/player when the table is "hot" or when it's time to leave.
Her relationship with Ben at first is "business only" but then they become romantically involved.
While card-counting is not illegal, the casinos have this funny concept that they are the ones to make a profit.
One of the casinos they've hit often has a security team headed by Cole Williams (Laurence Fishburne) who has been known to take card-counters to the basement and demonstrate with blows to the head and body why card-counting is not a healthy pastime.
During this time Ben's persona is changing. He has distanced himself from the friends he had before entering the card-counting group and begins living the Las Vegas gambler lifestyle. His relationship with Jill seems to be over. Then one night he allows his emotions to control his betting actions, and loses a bundle. Rosa goes berserk and throws him off the team and takes back all the money Ben was saving for Harvard.
With Ben's young life seemingly in ruins the only chance at redemption and to solve all his problems is one final big score against the casino with Rosa actively involved.
The intricacies of card-counting are explained in some detail and if you are not a numbers person, or enjoy playing cards you might find this boring. But the suspense is riveting and the acting by heavyweights Spacey and Fishburne is what you would expect -- excellent!