It starts in Yen Sid’s room, Yen Sid sits in his chair behind his desk, Sora and Riku stand across from him. They appear in their KH2 outfits, but Riku’s hair is short.

Yen Sid: From here, you two will take the Mark of Mastery Exam.
Sora, surprised: Even without taking it, no one can beat me!
Riku, facing downward: Having walked the path of darkness, I wonder if it’d really be right for me to wield the Keyblade. You’d have to make sure.
Sora: Riku…
Yen Sid: Let’s open the keyhole of sleep. If you return back to this place, then I will consider you Masters.
Sora on a balcony, opens his eyes.
Sora: Huh, where is this place? My clothes changed! More importantly, where’d Riku go?
Sora leans over the balcony and calls out in a loud voice: Heeeey! Riiiikuuu!
A mysterious boy descends from a rooftop in ambush: You’re so loud, stupid Noise.

Sora is surprised and jumps off the balcony.

Neku: You’re Sora…
Yen Sid: A large number of Heartless reside in the world of darkness. Instead, in the world submerged in sleep there exists a particular key. That is, creatures who consume dreams known as the Dream Eaters. Searching for the Dream Eaters will guide you to the key of sleep.
(Battle action sequences of Sora are shown.)
Neku, showing Sora the scarlet mark on his hand: This is from the Game. All the participants have it engraved upon them. I can’t afford to lose this Game!
Sora: I don’t really get ‘the Game’, but maybe I can lend a hand?
The scene changes to the world of Notre Dame.
Riku is speaking with Quasimodo who is holding Esmeralda: Quasimodo, where did the creatures go?
Quasimodo: To the rooftop of cathedral.
A Dream Eater with wings like a butterfly descends upon them.
Quasimodo: Me too, me too, I’ll fight too!
The scene returns to Neku and Sora in a plaza inTraverse Town.
Sora, facing Neku: Come to think of it, I didn’t ask your name.
All of a sudden several Dream Eaters appear around them!
Neku with surprise: Dream Eaters!
Sora: That’s a weird name.
Neku, pointing at the Dream Eaters: Not me, those!
Yen Sid: Opening the keyhole of sleep, along with finding a new power, the power locked in sleep will become revealed as well.

The trailer continues, showing the AR capabilities of playing with the Dream Eater pets, it’s a lot like Nintendogs. Maybe you’ll be able to raise them up too?

In a laboratory somewhere, a researcher has collapsed.
From this person’s POV, Xigbar becomes visible.

Xigbar: Xehanort! Have your memories returned?

From the view of the person referred to as Xehanort, a Keybalde appears before his eyes.

The screen then blacks out theatrically, displaying KINGDOM HEARTS 3D Dream Drop Distance, Spring 2012.