Cartoon Network Universe: FusionFall is a massively multiplayer online game developed by Cartoon Network and Grigon Entertainment. FusionFall uses the Unity engine as its client technology basis. Though the full game requires a subscription, free accounts can be registered which allow access to a portion of the content. It was originally scheduled to be released in Summer 2008, but the release date was pushed back to January 14, 2009.

Planet Fusion, ruled by the evil Fuse, has traveled throughout the galaxy for millennia, devastating countless planets and civilizations. Earth is its next stop. The player joins forces with the characters from various Cartoon Network series to repel this threat.

The game begins with the player character participating in Dexter`s time traveling experiment. As Dexter starts up the machine, Dee Dee, Dexter`s sister, sneaks in and messes with the controls. This causes a time-jump error, sending the player much further into the future than Dexter intended, where they arrive in the midst of a war between the Fusions and Cartoon Network`s characters. This section of the game serves as a tutorial, introducing the player to the controls and types of items in the game. After completing an optional training level, players are tasked with repairing Dexter`s time machine to return to the present. Upon gaining four Nanos and completing a number of key missions, the time machine is considered fixed and paid subscribers can return to the present for the main portion of the game.

Characters from various original Cartoon Network shows appear in the game, redesigned in an anime style by Mario F. Piedra. In addition, small, miniaturized versions of Cartoon Network characters called Nanos are collected along the way and travel with players, providing powers and abilities to assist during missions.

In FusionFall, the player takes control of a customized avatar to traverse the world. Their character is viewed from a third-person perspective, and the range of movement consists of walking in any direction and a short-distance jump. The game has large playable environments, with travel facilitated by various warp points. Gameplay primarily focuses on defeating fusion monsters for various reasons. The player is provided with numerous weapons to accomplish this, including projectile weapons (pistols, rifles, shotguns, and rockets), melee weapons, and grenades. Unlike ordinary role-playing games, the player does not gain experience in the conventional sense. Instead, they gather a loose analog known as fusion matter (which has multiple purposes) through defeating enemies and completing missions, and upon collecting enough can perform a mission to raise their level. The player is guided in their quest by a specific character, beginning with Dexter`s computer, Computress. After leaving the first level, they can choose between a wider range of guides, and can switch between these guides at their discretion for a small fee. The main focus of the game is the completion of missions. Missions are given by various characters (for example, Numbuh 5 of Codename: Kids Next Door and Ben Tennyson of Ben 10: Alien Force) around the game world, who usually cluster around travel hubs. Missions come in three types: Nano missions, Guide missions, and World missions. Nano missions are granted automatically once the player gathers enough fusion matter. A meter encircling the minimap shows the player`s progress in this regard. Guide missions are special tasks done on the behalf of the guide character, typically offering the best equipment available as a reward. World missions are generic tasks which offer lesser rewards, but are far more numerous to compensate. Unlike the first two types of missions, which can only be done one at a time, the player can run four different World missions concurrently.