"21" is a film about six MIT students learning how to count cards and then take down Vegas for millions at blackjack. The one thing you need for that is massive intelligence, yet that is the characteristic most absent from this film.
It's based on the true story and book "Bringing Down the House" and goes beyond the old standby that says the book is always better than the film. Changes were made, the least of which was making the main character white instead of Asian. Ben Campbell (Jim Sturgess) is an MIT student struggling for money. He needs $300,000 for medical school, yet never once considers a student loan. He's also never heard of blackjack, which almost makes less sense.