When the ThunderCats awake from their suspended animation on Third Earth, Lion-O discovers that his suspension capsule has slowed rather than stopped his aging, and he is now a child in the body of an adult. Together, the ThunderCats and the friendly natives of Third Earth construct the Cat's Lair, their new home and headquarters, but before long, the Mutants have tracked them down to Third Earth. The intrusion of these two alien races upon the world does not go unnoticed, however; the demonic, mummified sorcerer, Mumm-Ra, recruits the Mutants to aid him in his campaign to acquire the Eye of Thundera and destroy the ThunderCats so that his evil might continue to hold sway over Third Earth.