I think the Wikipedia summary will be more than enough. All I remembered going in was that there was a sophisticated software program that escaped from a Russian submarine, it was outguessing Tom Cruise's moves and a key (in two interlocking parts) was needed to somehow shut it down. That was enough, and the fluff they added in to pad out the movie includes flashbacks to previous movies... basically the franchise seems to be claiming that Final Reckoning is a sort of grand unifying movie tying up _all_ the previous movies into one grand conclusion. I don't think you need to rewatch all the previous movies. To me that's just a bit of pop spiritualism; like Tom Cruise's character, we are all the sum total of our experiences, it's not a revelation.