OK, I’m sure all this jumping back-and-forth in time in the Kingdom Hearts games must have people confused, so here is a timeline (far from complete, but it still contains spoilers) that I put together; hope it helps.
Birth By Sleep: Ten years before KH One.
-Introduces three new heroes, Ventus, Terra and Aqua- all keyblade users (apparently the ones from the hidden KH videos.)
-Shows how Mickey Mouse met Ansem.
-Shows how Xehanort became Ansem’s student (by stealing somebody else’s body!)
-Sora, Rikku and Kairi show up- as toddlers.
At Some Point Before KH1- not shown anywhere that I know
-Ansem finds out that his students are researching The Darkness against his wishes; Xehanort banishes him (and starts taking over his identity).
-Their experiments turns the students into Nobodies; they form Organization 13.
-The Heartless are unleashed into the universe, and they begin multiplying and destroying worlds.
Kingdom Hearts One:
-Sora becomes a Keyblade Wielder, and starts fighting The Heartless.
-Goofy and Donald join him on Mickey’s orders, while Mickey himself goes on his own mission.
-Disney Villains (and Xehanort, or his Heartless pretending to be Ansem) kidnap the Disney Princesses, seeking to use their hearts (souls?) to open Kingdom Hearts, the door to the center of the Universe.
-Rikku joins the bad guys, out of jealousy; however he later switches sides.
-In a sequence of events I’m still not clear on, both Sora and Kairi spawn alternate selves known as Roxas and Namine (without being aware of them.)
-The villains are defeated and the universe reforms. However, both Rikku and Mickey are trapped behind Kingdom Hearts with an army of Heartless.
Chain Of Memories:
-Sora, Goofy and Donald are led to Castle Oblivion (by Org. 13) where they slowly lose their memories (this is done by Namine, under coercion from Org. 13.)
-Rikku, with Mickey’s aid, saves them, but they lose all their memories and must rest for a year to recover them.
358/2: (note, this begins before CoM and ends almost year later)
-Roxas joins Org. 13.
-He later rebels rebels against them, and several members are destroyed.
Kingdom Hearts 2:
-Roxas, who has no memories of the previous events, is trapped in a virtual reality by the man called DiZ (who is actually Ansem) while the sleeping Sora slowly reabsorbs him.
-Sora, Goofy and Donald awaken with no knowledge of the missing year (or of Roxas.). Sora then goes back to fighting the Heartless, unaware that that is exactly what Org. 13 wants him to do (since the hearts stolen by the destroyed Heartless form a new Kingdom Hearts.)
-The surviving members of Org. 13 are destroyed.
-Sora and Rikku are reunited. as are Mickey, Donald and Goofy.
Kingdom Hearts: Coded (Japan-only cell phone game) Takes place right after KH2.
-Mickey enters Jiminy Cricket’s journal (which works as a virtual reality) to fight Maleficent and Pegleg Pete with aid from a Virtual Sora.
Aaand that’s all for now.
Things I’d like to know (no, I haven’t played all the games yet):
-What happened to the original three heroes?
-What was Mickey’s mission?
-How did Mickey and Rikku escape Kingdom Hearts?
-Are there Heartless of Org. 13?
-Or, for that matter, of Sora, Kairi (and maybe Rikku?)