Now this is something which I never quite understood when I was a believer, the claim that the bible is all one big story, or that as a whole it conveys one unified message. If there were only 1 message in a book that size, it would be the king of boring books, nothing would ever be able to touch it in boring-ness.
I think that the reason this 'argument' has risen to popularity lately is perhaps because many Christians can see the blatant discrepancies between biblical texts, so they unconsciously concede the point that the bible is full of contradictions. They retreat to the position that "It's all one story", as if that is some kind of argument for its validity (which it most certainly isn't). The only possible way I can see of linking up all the books of the bible together would be the following summary "God does some things". In Genesis he's creating things, sending snakes to lie to people, promoting incest, committing genocide and giving boring genealogies. In Exodus he's talking through trees, sending plagues, murdering children, turning rivers into blood, turning sticks into snakes, drowning entire armies and kicking people out of their homelands. I think you get the point by now. The 'story' can only be unified by the line "God does things", and to suggest otherwise would be dishonest.
You could possibly elaborate on that by saying "God flaunts his power, and might makes right" or something like that, but I can't imagine the overall plot line would get any more complicated than that.