Remember how Terra was taken over by Master Xehanort, well he takes over Aqua instead. Terra and Ven battle Aquanort, but Ven was hit by a dark blast and his heart left his body. Terra continues the battle and breaks the bond between Xehanort and Aqua. Terra thern allow xehanort's free heart to enter his body and seaked it away and, Ven's Bpdy, and Mickey are in Yen Sid's tower. And the rest is history: Aqua takes Ven's body to Land of Departure which Becomes Castle Oblivion, Aqua falls sleep in Ven's Chamber, but wakes up 11 years later to find Sora, Riiku, Kairi, Roxas, and Namine there in the chamber with her and Ven. It sets post DDD, so Riku and Sora are masters. They wake up Ven and goes back to Radiant Garden to meet with Leon and the others. Aqua's eyes flash blue to yellow and Vanitas is revealed to be alive. That's the story in a nutshell.