Unless you have been playing Mortal Kombat since the very beginning, the storyline is a tad complicated. By complicated I mean you require access to wikipedia, a conspiracy theorist, some narcotics and a screwdriver to work out what exactly is going on. Then the game goes and retcons everything in the previous title anyway, making all that history null and void!
Okay it isn’t really that complicated, if you played the story mode of the last two games you should have a pretty solid idea of what is going on. Or you could just be playing to see as much blood on the screen as possible.