The Land of Ooo is the main setting of Adventure Time, and is the home of Finn and Jake, along with all of their friends and foes. It is divided into different kingdoms, of which the most prominent are the Ice Kingdom, Candy Kingdom, Fire Kingdom, Breakfast Kingdom, and Cloud Kingdom. There are also many geographical areas not known to be part of any kingdom, such as the Evil Forest and the Bad Lands.
In FusionFall, Ooo is falling apart. The Ice Kingdom has broken off from Ooo (literally, because the Ice King used his magic to transport his mountain to Orchid Bay). Natural disasters began to occur with terrifying frequency; earthquakes, tempests, and tornadoes have devastated the entire south of Ooo, forcing the inhabitants to flee northward. Princess Bubblegum had left the throne of the Candy Kingdom to track down the Ice King and then to join the anti-Fusion Resistance, and Lemongrab had become ruler until such time as Bubblegum would return. Due to Lemongrab's lack of political experience (and any kind of character or intelligence), the economy in the the Candy Kingdom plummeted. With so many kingdoms destroyed, every Kingdom has agreed to unite until the disasters would subside. Since most agreed that Bubblegum would be the best choice to rule a united Ooo, they decided to place Lemongrab on the throne as her vice-regent -- a bad decision, since he merely extended the economic disaster of the Candy Kingdom throughout the entire land of Ooo. Since South Ooo lay undefended, Fuse's army made it their base, and have established a puppet state there.