Sora goes into a slumber due to the forced hand of namine. A dream witch you could say. The organization that forced her is organization IIIX. During this slumber roxas, a part of sora who is a part of roxas awakens, namine is freed then goes about trying to awaken sora in 365 days hence 365/2days one of the games, and the prequel to KH2. Birth by sleep which i havent play is a prequel to KH 1 and introduces many behind the scene characters and helps set the stage for the keyblade wars-KH3. Also dream drop distance. Is the latest installment. And sets the final stage for the keyblade wars. I get so mad when people say this is a child's game because its disney. Yes it has some corny values, but apparently they have never played it.
Wiki it if you want to get the full story. Its taken me a month to even begin to understand whats going on.