Well, let me sum it up for you in words of Rayden... Earth is but one of many realms, one of them is a forsaken land called Outworld, where an immortal creature claimed itself the emperor... He wanted to take over earth, but the Elder Gods gave him a condition, he had to win 10 Mortal Kombat tournaments in a row... However, if earth realm won, the champion would retain his youth until the next tournament... That's the rules...

uh...those aren't really Rayden's words. Well, not all of them anyway.

Anyway, maybe this will help:

http://www.mkwarehouse.com/story/mk2.htm

http://www.mkwarehouse.com/story/mk3.htm

Those are links to sections of the Mk Warehouse(god bless it) explaining the storyline of MKII and MK3. You pretty much get the idea of MK overall if you read through those two.