Mortal Kombat started out somewhat managable.  It wasn't too difficult to keep track of characters like Scorpion, Sub-Zero, and Sonya Blade and their motivations.  Then Mortal Kombat II upped the cast from ten characters (seven of which are playable) to twenty-one (sixteen of which are playable).  The Mortal Kombat world only grew larger from there are sequels and variations changed the rosters, rules, and even the plot as whims and the market demanded.  It's downright near impossible to easily follow the progression of Mortal Kombat.  Thankfully, Hardcore Gaming 101 has put together an exhaustive and complete record of what the franchise has to offer including spin-offs, crossovers, and appearances in other media.

Somewhere on a far away island, a secretive martial arts tournament has been held for the last 500 years. The mastermind behind this tournament, Shang Tsung, is actually working for the emperor of an alternate dimension known as Outworld. This tournament is the only thing stopping the forces of Outworld from crossing into our dimension and conquering it completely. The good news is that for that to happen, Outworld has to win ten tournaments. The problem is that they've won nine, thanks to the help of their four-armed champion, Goro. And so, as one of seven warriors who've ended up on Shang Tsung's island by either destiny or by accident, it's up to you to make your way through the tournament and defeat the rest of Earth's champions. Do that, and you'll have to go against Goro, before fighting against Shang Tsung himself for the fate of the world. This is, of course, if you don't find yourself having your spine extracted by a pissed-off ice ninja along the way.

While Mortal Kombat was red hot in the 1990s, it started to lose buzz following Mortal Kombat 3 and the already spiraling story became even more convoluted.  A series of lackluster, rushed ports of the arcade games to home platforms didn't help either.  Lapsed fans may have missed that Freddy Kreuger of A Nightmare On Elm Street fame visited Outworld or that the heroes and villains of the DC Universe clashed with Shao Kahn's forces.  I'll let MetaFilter's clockzero sum up the relevant thoughts that occured while reading about how much Mortal Kombat has grown over the years:

Today I learned that they're apparently still making Mortal Kombat sequels.

I just read the first part, the characters section.  It went like this:

Uh-huh, Liu Kang.  Scorpion.  Sub-Zero, ha, Reptile.  I remember these guys.

Oh, that dude's name was Goro? Huh. 

Kitana, eh?  I guess they added female characters.  Ha.  Great name, guys.

Shao Kahn, the boss!  Wow! 

Oh jesus, Nightwolf?  Wow.  Okay.  Native Americans can have bad names I guess.

Rain?  Chameleon?  ERMAC?  WTF?

Kai? Reiko?  MEAT?  How many years has it been?  FUCK do I feel old.

Okay, they...made a character who's a motion capture actor.  That's okay, that's...normal.  There can't possible be much more.

Oh, there's like, five more pages of characters...

Freddy...Krueger.  And Lex Luthor!  Why the fuck not!

It was at that moment that I realized that someone, somewhere, was fucking with me.

That someone is Ed Boon and he does that sort of thing for a living. Thankfully.