Riku decides to run off with Ansem with Kairi's pretty much dead body
While Sora gets blasted off to Traverse Town meets up with Squall--I MEAN LEON, Yuffie, Cid, And Aerith
  And Sora's all I HAVE TO FIND MY FRIEND
And Donald and Goofy are all WE HAVE TO FIND OUR KING
  And then HEARTLESS FUCKING EVERYWHERE
SO they team up to find their friends
Meanwhile, Sora gets this super magical weapon called the Keyblade
Which can unlock people's hearts, whatever that means, and seal and open magic portals to other worlds

So they hop on the dope-ass Disney spessship and blast off into Disney Spess
They hop from world to worl, looking for Mickey, Riku, and Kairi, fighting Heartless and sealing the magical portals to the very cores of the world
Because if the Heartless reach the heart of the world, it blows the fuck up
So eventually they track down Riku and Kairi to Hollow Bastian
Which is this really fucked up castle where waterfalls fall up
And they seal the door to Kingdom Hearts

You go and fuck up bitches with Beast, and you pretty much have to let him do the fighting because you have a wooden sword and are useless as balls
Eventually, you get your shit back, learn about what the fuck's going on, and fight Riku who's totally Ansem now
And then you find out that Kairi's heart was inside of you all along and that you have to stab yourself with your Keyblade to get her out
so you do
 And then you become a Heartless for a while

But first you seal the keyhole of Hollow Bastian
And then Kairi uses her Mary Sue powers to I DON'T CARE