The series plot begins with the dying planet Thundera meeting its end, forcing the ThunderCats to flee their homeworld. The fleet is attacked by the Thundereans’s enemies, the Mutants of Plun-Darr, who destroy most of the starships in the “ThunderFleet,” but spare the flagship hoping to capture the legendary mystic Sword of Omens they believe is on board. The sword holds the Eye of Thundera, the source of the ThunderCats’ power, which is embedded in the hilt.
When the ThunderCats awake from their suspended animation on Third Earth after ten “galacto-years,” Lion-O discovers that his suspension capsule has slowed, rather than stopped, his aging. He has now become essentially 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, as a demonic, mummified sorcerer calling himself 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 may continue to hold sway over Third Earth