There is a mode to tie all this abject violence together; it's called Konquest. This single-player, action/adventure romp through the many realms of Mortal Kombat introduces brothers Taven and Daegon, who have just awoken from a lengthy sleep to fulfill a prophecy and blah blah blah. The writing and acting are equally cheesy, but the potential for carnage is unrivaled - you'll slice through dozens of bad guys and even take out a good number of the main MK characters through the roughly eight-hour journey, picking up invisible icons along the way. That's annoying, but it's worth playing, especially since collecting koins helps you pimp out your created fighter.
You'll also meet and beat a few dozen of the marquee characters along the way, though it is a bit awkward to constantly switch between the one-against-an-army Konquest controls and the one-on-one Kombat controls, where some of the buttons do completely different things.