From the introduction:
Welcome to Third Earth a strange and dangerous planet inhabited by countless different species and home to many dark & deadly secrets. The cat-like Thunderians rule over many of the planet's other animal-like inhabitants while selfishly keeping much of the world's resources and fertile land for their own kingdom. Though a combination of magical knowledge, military power, economic prosperity, and access to strategic resources, the cats have become the greatest power in all the known world, something that they're keen to make sure the other species remember. However, unbeknownst to the rulers of Thundera, another species has finally gained the means to challenge their feline masters and very soon they, along with an ancient evil long thought defeated, will strike a devastating blow to the very heart of the Thunderian empire. You will arrive one year before the city of Thundera is destroyed and Mumm-Ra nearly wipes out all the cats, causing the young king Lion-O to begin his quest to defeat the evil sorcerer.