All you really need to know about 21 is shown in the (excessively) stylish opening prologue before the credits. It's a foretaste of what's to come later in the film—not just the general plot details, the narration, and the visual style, but even some of the "how" that will bring us to that point. You could say 21 reveals its cards too early.
Not that you haven't seen movies like this before. It's become a genre unto itself in the last 20 years, which, for now, I'll refer to as MMM—the Misleading Mentor Movie. Young man looking for big break struggles to make ends meet, becomes persuaded by charismatic Svengali to enter the high stakes world of [fill in the blank], only to confront his core of ethics amidst his runaway success. We've seen it applied to a numerous trades: the stock market (Wall Street), bartending (Cocktail), pool hustling (The Color of Money), investments (Boiler Room), and sports betting (Two for the Money), to name a few. Here, it's set in the world of casino blackjack.
21 is based on Ben Mezrich's best-selling book Bringing Down the House, which in turn was inspired by the true story of Jeff Ma, who, along with a team of fellow M.I.T. students, developed an intricate system for card counting in blackjack. Contrary to popular belief, card counting is not illegal—provided no added technology or outside device is being used—but the strategy is strongly discouraged by casinos since it can lose them lots of money. In the case of Ma and his buddies, they were beating casinos for hundreds of thousands of dollars at a time.
A premise with strong potential, but here it's applied to familiar MMM conventions. Jim Sturgess (Across the Universe) sheds his British accent to play Ben Campbell, a pre-med student at M.I.T. who has been accepted to Harvard Medical School. Problem is, he doesn't have the money to follow through on his dream—certainly not by working at the local men's clothing store. But his math and memory skills attract the attention of his charismatic professor Micky Rosa (co-producer Kevin Spacey), who invites Ben to join his team of specialized students, conveniently including Jill Taylor (Kate Bosworth), whom Ben has a crush on.
Together, they head to Vegas on weekends with a setup worthy of Mission: Impossible. The students play blackjack conservatively at various tables, monitoring the number of high cards left in the deck. When the table is hot—rife with tens and aces —they signal their "big player," wandering between tables to place high bets at the right time. This is where the film is engrossing, demonstrating the tricks of the trade.
As proven by Ma and his cohorts back in the mid '90s, the strategy works—unless, of course, the casino finds you out. In real life, the MIT students were simply asked to leave when caught by casino security, and they eventually gave their scheme up with the development of video technology that runs faces through a computer database of known counters. In the film, security man Cole Williams (Laurence Fishburne) hauls counters into the casino basement and discourages them from playing by pummeling them.