This movie operates on a supernatural premise: that luck is real, and that people can visit luck, good or bad, upon you. And where else would this be more useful than a Vegas casino?
William Macy plays a personification of bad luck, who is "gifted" with the ability to shift the odds in the casino's favor. Maria Bello, through complex circumstance, turns things around π radians for him.
This would normally be the stuff of wistful romantic comedy, save for Alec Baldwin, who plays the old-style casino manager. Old-style in the sense that he's got few scruples about having people beat up or murdered. And he doesn't take Macy's sudden turnaround well at all. The result is relatively unwistful.