ThunderCats follows the adventures of the eponymous team of heroes, catlike humanoid aliens on a planet called Third Earth. The series plot begins with the dying planet Thundera meeting its end, forcing the ThunderCats (a sort of Thunderean nobility) 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's power, which is embedded in the hilt. Though the Mutants damage the flagship, the power of the Eye drives them back. The damage to the ship means the journey to their original destination is not possible, instead having to journey to "Third Earth;" which will take much longer than they had anticipated. The eldest of the ThunderCats, Jaga, volunteers to pilot the ship while the others sleep in capsules; however, he dies of old age in the process, but not before ensuring they will reach their destination safely. The flagship contains the young Lord of the ThunderCats, Lion-O, as well as the ThunderCats Cheetara, Panthro, Tygra, WilyKit and WilyKat, and Snarf. 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. (He has thus grown in size without actually growing up, and has missed out on the first-hand life experiences which confer maturity.) 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.