The video begins with Master Xehanort describing the keyblade legend to Master Eraqus. Terra, the boy in blue, dreams of being a keyblade master and he has a dark side that Xehanort is ready to exploit.
While watching the trailer, keep an eye out for kid versions of other Kingdom Hearts characters. Kairi, Riku, Axel, etc. appear briefly and interact with the three stars in Birth by Sleep.
Wait… did I just see Sora at the end?
Yeah, that looks like Sora dressed in his Kingdom Hearts 2 outfit.