If you start KH2 playing as Sora, it HAS to start with him getting OUT OF the rose pod... Otherwise, Donald and Goofy are going to be carrying that thing around everywhere they go... ><
Anyway, okay. I'll tell you!
Sora, Goofy, and Donald are walking on the same path they were on at the end of KH1.
Then, they see a castle: Castle Oblivion. It has a bunch of unknowns.
They all go in, and Sora fights the Organization Members. Guys in cloacks and stuff. They're supposedly in cahoots (Yes, I just said that) with Ansem, who may or may not be DiZ (this one dude wrapped in red cloth).
Sora goes from floor to floor, going to the worlds he's already been to. They're made from his memories, but he doesn't know how (He usually fights one org. member per floor). He forgets Kairi and "remembers" this blonde chick Namine. He's convinced that he has to save her, and he and Riku are competing for HER attention. In the end, Sora SEEMS to defeat the Organization... Mostly, kinda, sorta, and he gets to Namine. Sora finds out that Namine is NOT the girl he thinks she is; she's been manipulating his memories. She's a witch. She was ordered by the Org. to erase Sora's memories and get him to trust her. He's at the end now, and to continue, he has to decide to keep the memories he made in Castle O., or go back the way he was.
He decides to go back to the way he was, and he and Namine swear to find each other and be friends or whatever, so he'll remember her EVENTUALLY. Namine puts him in this weird, Rosebud-Pod-thing, where he sleeps for a year to get his memories back. Donald and Goofy do that too (they also had a fight for a few floors, and Sora had to fly solo, but then they were friends again).
And that is a SHORT summary.... Weird, huh?