The brainiacs of the gambling romp “21” are smart enough to expertly count cards at the blackjack table, identify hot betting tables and put on disguises so they can take Vegas casinos to the cleaners.

So why are these MIT scholars so dumb they fall into greedy, grubby plot holes a C-minus Statistics 101 student would have seen coming a mile to the Nth power away?

The movie’s a morality play preaching sophomoric ethics – avarice, bad, clean living and hard work, good. Yet in doing so, the only interesting thing “21” holds up to the light is the gluttony the movie eventually decries – money, booze, fast living, the sheer intemperance of making a killing, Vegas-style.

The characters are reprehensible at worst, seedy and selfish at best. We’d all love to cash in the way they do and get a taste of the action, but it makes for a thin night at the movies to watch dreary strangers do it in our place.

Moving from romantic comedy into supposedly heavier material, “Legally Blonde” director Robert Luketic takes on this adaptation of Ben Mezrich’s book “Bringing Down the House,” an account of real Massachusetts Institute of Technology students who beat Vegas at the blackjack tables.

The dramatization of their exploits centers on Ben Campbell (Jim Sturgess, the crooning romantic lead of last fall’s “Across the Universe”), a math whiz who needs either a free-ride scholarship or $300,000 hard cash to move on to Harvard Medical School.

With the scholarship looking iffy, how convenient that one of his professors, the flamboyant Micky Rosa (Kevin Spacey), approaches Ben to fill an open spot on his blackjack team, an extracurricular activity for select students who spend weekends in Vegas winning barrels of cash by counting the cards to determine the probability of winning hands.

It’s also convenient that a lovely classmate the slightly geeky Ben already has his eye on, Jill Taylor (Kate Bosworth), is a member of the blackjack team. (Care to predict the probability they’ll end up in bed together on one of their Vegas visits?)

With Micky pulling the strings, Ben, Jill and their confederates cash in with a vengeance before mean old Mr. Greed makes them all start behaving like oafs.

Luketic and screenwriters Peter Steinfeld and Allan Loeb bring no grace or subtlety to the shift from team spirit to me-first the characters undergo. Midway through the movie, they simply decide that the fun’s over and everyone has to start turning on one another.

The part of the heavy goes to Laurence Fishburne as Cole Williams, a tough guardian for the casinos who does not shy from using his knuckles to discourage card-counters at his establishments. The filmmakers cannot decide whether to present Cole as a good or bad guy, so in the end, they just make him another covetous soul looking out for himself.

Spacey’s fun to watch the first half of the movie as Micky mentors his proteges on the joys of self-indulgence, but an abrupt switch in his demeanor is shallow and unconvincing. And we never do learn why it is that with the foolproof system Micky’s been employing for decades, he’s still toiling in the classroom rather than sipping boat drinks in the Bahamas.

This is the third movie in short order in which Spacey and Bosworth have co-starred, following “Beyond the Sea” and “Superman Returns.”

Bosworth is little more than window-dressing here, while Sturgess is merely adequate as the naive young genius seduced by corrupt influences.

The production has plenty of style and flash, but the Vegas imagery is obvious and rather tired from overuse after all those episodes of the original “CSI” and the “Ocean’s Eleven” movies.

Lack of originality is where “21” really deals a losing hand.

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