I would also suggest Wikipedia, but Kantor gave the very general outline of the story from the first game.
*POSSIBLE SPOILERS*
@ Kantor, there is strong evidence in the second game (Altair's journal thingy) that says he took the Piece of Eden. It's the whole Codex page thing. Altair talks about some of the things he tried using it for. I believe one of those things was allowing him to make the Armor of Altair that's so helpful in AC2. It's been a while since I read through all of that though.
The second game was a blast, and the platinum trophy is a pretty easy one to get.