While exploring Hollow Bastion in Kingdom Hearts, you were provided the ability to have Beast in your party. He was searching for his kidnapped love, Belle who was one of the princesses of heart. In Kingdom Hearts 2, Sora & Co. will journey to the homeland of Beast & Belle.
The world starts off with Sora & Co. entering the castle to find a Shadow Heartless. The group decide to follow it throughout the Castle. Later, more Shadows appears but then Beast arrives and kills them. When the group attempt to talk to Beast, they get attacked by him. Beast then takes the nearby Rose in the room and dissapears.
Later, you meet Belle who explains that Beast has imprisoned the entire castle's staff in a dungeon. Belle is obviously concerned and pretty scared, which leads Sora to offer help. Journeying throughout the castle leads to the group finding a Dresser who is not prisoned, he infoms them of the staffs location. Upon arriving, the group have to face off against a boss in the form of a door.
Present in this world is Xaldin, who manipulates Beast (explaining his agressive behaviour) and then later, kidnaps Belle. When Beast returns to his self, he teams up with Sora to fight against the Nobody's summoned by Xaldin, in addition to fighting him.