Sora, Donald, and Goofy have been asleep for the past year to regain their lost memories. Roxas, Sora's Nobody, is trapped in a virtual simulation of Twilight Town created by DiZ to merge him with his original self and restore Sora's power. DiZ's plans are threatened when Nobodies led by Axel, Roxas's former friend in Organization XIII, infiltrate the virtual town to extract Roxas. However, Roxas is able to repel the Nobodies and merge with Sora. Sora, Donald, and Goofy awaken in the real Twilight Town and meet King Mickey and Yen Sid, who send them on another journey to find Riku and uncover the Organization's plans. Afterward, Maleficent is resurrected and joins with Pete to continue her quest for power.
The player directly controls Sora from a third-person camera angle, though first-person perspective is available via Select button. Most of the gameplay occurs on interconnected field maps where battles take place. The game is driven by a linear progression from one story event to the next, usually told via cutscenes, though there are numerous side-quests available that provide bonuses to characters. Like many traditional role-playing video games, Kingdom Hearts II features an experience point system which determines character development. As enemies are defeated, the player and allies gain experience to "level up", allowing them to grow stronger and gain access to new abilities.
Source: Wikipedia, "Kingdom_Hearts_II," available under the CC-BY-SA License.