Set after the events of Kingdom Hearts Re:coded, the game focuses on Sora and Riku's Mark of Mastery exam in which they have to protect parallel worlds in preparation of the return of their enemy Xehanort. Besides controlling the two playable characters across a single scenario, the player is able to recruit creatures known as Dream Eaters able to assist them in fights.
The two playable characters are Sora and Riku, control of whom rotates throughout the game through the use of a gameplay element called the "Drop" system. This system features a Drop Gauge which gradually depletes on its own over time, automatically switching control between Sora and Riku once it empties. The player is able to keep the gauge from emptying by attacking enemies, and can switch characters at will using save points. An item called "Drop-me-not" can be used to replenish it as well. The amount of time the gauge can be leveled up if the player uses "Drop Points" when switching between characters.
The game sees the return of the "Command Deck" system which was introduced in Kingdom Hearts Birth by Sleep and re-utilized in Re:coded. One of the key elements of the game is the "flowmotion" mechanic, allowing the player to utilize the surroundings for movement actions, and attacking with the execution of the Y button. When the player is using flowmotion, Sora or Riku will be covered in a violet aura, signifying its use. The game's most significant feature is the ability to hunt and collect Dream Eaters, with each individual Dream Eater companion providing Sora and Riku access to varying abilities, attacks, and cooperative actions. Many elements of the game utilize the touch screen, such as throwing enemies or chaining various attacks. The game also has interactive cut-scenes and events. Unlike previous games which used the Gummi Ship as a means of travelling to other worlds, this game makes use of a new mini-game called Dive Mode. During Dive Mode, the character floats towards the following world's gate that can only be opened by collecting a specific number of stars in the first one. In following Dive Modes the player encounters enemies that can be stopped by using items that can be found while floating. Some Dive Modes have a Dream Eater boss that needs to be defeated instead of collecting stars.
Source: Wikipedia, "Kingdom Hearts 3D: Dream Drop Distance", available under the CC-BY-SA License.